Electroluminescent device, and display device comprising thereof

ABSTRACT

An electroluminescent device includes a first electrode and a second electrode facing each other; an emission layer disposed between the first electrode and the second electrode and including a plurality of quantum dots and a first hole transporting material having a substituted or unsubstituted C4 to C20 alkyl group attached to a backbone structure; a hole transport layer disposed between the emission layer and the first electrode and including a second hole transporting material; and an electron transport layer disposed between the emission layer and the second electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-In-Part application of U.S. patentapplication Ser. No. 16/562,601, filed on Sep. 6, 2019, which claimspriority to and the benefit of Korean Patent Application No.10-2018-0107005 filed in the Korean Intellectual Property Office on Sep.7, 2018, and all the benefits accruing therefrom under 35 U.S.C. § 119,the entire contents of which are incorporated herein by reference.

BACKGROUND 1. Field

An electroluminescent device and a display device including the same aredisclosed.

2. Description of the Related Art

Quantum dots are nanocrystal semiconductor materials having a diameterof less than or equal to around 10 nm, and show quantum confinementeffects. Quantum dots generate stronger intensity light in a narrowerwavelength region than the commonly used phosphor. Quantum dots emitlight while the excited electrons are transited from a conduction bandto a valence band and emission wavelengths vary depending upon aparticle size of the quantum dots. Quantum dots can therefore be used toobtain light in a desirable wavelength region by adjusting the size ofthe quantum dots.

Advantages to electronic devices having an emission layer includingquantum dots include reduced production cost, as compared to the organiclight emitting diode (OLED) using an emission layer includingphosphorescence and/or phosphor material. In addition, different colorsmay be emitted by changing the size of the quantum dots, whereas OLEDdevices require the use of different organic materials in the emissionlayer for emitting different colors of light.

The luminous efficiency of the emission layer including quantum dots isdetermined by quantum efficiency of quantum dots, a balance of chargecarriers, light extraction efficiency, and the like. Particularly, inorder to improve the quantum efficiency, the excitons may be confined inthe emission layer, but when the excitons are not confined in theemission layer by a variety of factors, it may cause a problem such asexciton quenching.

SUMMARY

Provided are an electroluminescent device having improved devicecharacteristics by improving hole transporting capability and surfacecharacteristics of an emission layer and a display device including thesame.

According to an embodiment, an electroluminescent device includes afirst electrode and a second electrode facing each other; an emissionlayer disposed between the first electrode and the second electrode andincluding a plurality of quantum dots and a first hole transportingmaterial. The electroluminescent device may further include a holetransport layer disposed between the emission layer and the firstelectrode, and including a second hole transporting material; and anelectron transport layer disposed between the emission layer and thesecond electrode, or both of them.

The quantum dots are configured to emit a first light on excitation bylight and the first hole transporting material is configured to emit asecond light on excitation by light. The first light has a first(maximum) peak wavelength and the second light has a second (maximum)peak wavelength. In a time resolved photoluminescence spectroscopyanalysis of the emission layer, t1 for the first light may be less thanor equal to about 15 nanoseconds (ns) or less than or equal to about 10ns.

In the time resolved photoluminescent spectroscopy analysis of theemission layer, a decay fraction of the t1 may be greater than or equalto about 5%, greater than or equal to about 10%, greater than or equalto about 15%, greater than or equal to about 20%.

In the time resolved photoluminescent spectroscopy analysis of theemission layer, an average decay time (t_(avg)) for the first light maybe less than or equal to about 30 ns, or less than or equal to about 27ns.

In the time resolved photoluminescent spectroscopy analysis of theemission layer, an average decay time (t_(avg)) for the second light maybe less than or equal to about 0.7 ns.

In an analysis for an excitation wavelength dependent photoluminescenceemission behavior, the emission layer may show a luminance intensitythat is less than or equal to about 0.4, less than or equal to about0.3, less than or equal to about 0.2 of a maximum intensity thereof,when an excitation light has a wavelength of the second peak wavelength.

In the analysis for an excitation wavelength dependent photoluminescenceemission behavior, the emission layer may show a luminance intensitythat is less than or equal to about 0.2, or less than or equal to about0.1 of a maximum intensity thereof, when an excitation light has awavelength of 450 nm.

The second peak wavelength may be less (e.g., shorter) than the firstpeak wavelength.

A difference between the second peak wavelength and the first peakwavelength may be greater than or equal to about 10 nm.

A difference between the second and the first peak wavelengths may begreater than or equal to about 150 nm.

When a voltage is applied, the emission layer may emit light of the samecolor as the first light.

The emission layer may be configured to emit light without a formationof a triplet emitter. The emission layer may not include an iridiumcontaining organic metal compound, a platinum containing organic metalcompound, or a combination thereof.

The first hole transporting material may include a substituted orunsubstituted C4 to C20 alkyl group attached to a backbone structure.

The first hole transporting material may include a compound representedby Chemical Formula 1.

In Chemical Formula 1,

R¹ to R⁸ are each independently hydrogen, a C1 to C3 alkyl group, asubstituted or unsubstituted C4 to C20 alkyl group, a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, a substituted or unsubstituted C3 to C20heteroaryl group, a substituted or unsubstituted C2 to C40 alkylaminegroup, or a substituted or unsubstituted C6 to C40 arylamine group,provided that at least one of R¹ to R⁸ is a substituted or unsubstitutedC4 to C20 alkyl group, and

if any of R³ to R⁸ are a substituted or unsubstituted C3 to C20cycloalkyl group, a substituted or unsubstituted C6 to C20 aryl group,or a substituted or unsubstituted C3 to C20 heteroaryl group, it may beeach independently fused with an adjacent aromatic ring to provide a C8to C15 fused ring,

X¹ and X² are independently selected from N and C(—R^(a)), and X³ and X⁴are independently selected from S, N—R^(b), and C(—R^(c))(—R^(d)),wherein R^(a), R^(b), R^(c), and R^(d) are independently selected fromhydrogen, a substituted or unsubstituted C1 to C20 alkyl group, asubstituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, and a substituted orunsubstituted C3 to C20 heteroaryl group,

L¹ and L² are independently selected from a single bond and asubstituted or unsubstituted methylene group or C2 to C4 alkenylenegroup, and

i, j, k, l, and m are independently 0 or 1.

The first hole transporting material may include a compound representedby Chemical Formula 2.

wherein, in Chemical Formula 2, X³, X⁴, L¹, L², j, k, l, and m are thesame as defined in Chemical Formula 1,

R¹ to R⁸ are each independently hydrogen, a C1 to C3 alkyl group, asubstituted or unsubstituted C4 to C20 alkyl group, a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, a substituted or unsubstituted C3 to C20heteroaryl group, a substituted or unsubstituted C2 to C40 alkylaminegroup, or a substituted or unsubstituted C6 to C40 arylamine group,provided that at least one of R¹ to R⁶ is a substituted or unsubstitutedC4 to C20 alkyl group.

If any of R³ to R⁶ are one of a substituted or unsubstituted C3 to C20cycloalkyl group, a substituted or unsubstituted C6 to C20 aryl group,or a substituted or unsubstituted C3 to C20 heteroaryl group, it may beeach independently fused with an adjacent aromatic ring to provide a C8to C40 fused ring.

At least two of R¹ to R⁶ may be a substituted or unsubstituted C4 to C20alkyl group.

The first hole transporting material may include at least one ofcompounds represented by Chemical Formula 2-1 to Chemical Formula 2-2.

In Chemical Formula 2-1 to Chemical Formula 2-2,

X³ and X⁴ are the same as defined in Chemical Formula 1,

R¹¹, R¹², R¹³, R¹⁴, R¹⁵, R¹⁶, R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are eachindependently hydrogen, a C1 to C3 alkyl group, a substituted orunsubstituted C4 to C20 alkyl group, a substituted or unsubstituted C3to C20 cycloalkyl group, a substituted or unsubstituted C6 to C20 arylgroup, a substituted or unsubstituted C3 to C20 heteroaryl group,provided that at least one of R¹¹ to R¹⁴ and at least one of R²¹ to R²⁶are a substituted or unsubstituted C4 to C20 alkyl group,

R²⁷ and R²⁸ are each independently hydrogen, a C1 to C3 alkyl group, asubstituted or unsubstituted C2 to C40 alkylamine group, a substitutedor unsubstituted C6 to C40 arylamine group, or a substituted orunsubstituted carbazolyl group, and

If any of R¹³, R¹⁴, R¹⁵, R¹⁶, R²³, R²⁴, R²⁵, and R²⁶ is one of asubstituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, or a substituted or unsubstitutedC3 to C20 heteroaryl group, it may be each independently fused with anadjacent aromatic ring to provide a C8 to C40 fused ring.

The first hole transporting material may include a compound representedby Chemical Formula 3.

In Chemical Formula 3,

X³, X⁴, L¹, L², j, k, l, and m are independently the same as defined inChemical Formula 1,

R³³, R³⁴, R³⁵, R³⁶, R³⁷, and R³⁸ are each independently hydrogen, a C1to C3 alkyl group, a substituted or unsubstituted C4 to C20 alkyl group,a substituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, a substituted or unsubstituted C3to C20 heteroaryl group, a substituted or unsubstituted C2 to C40alkylamine group, or a substituted or unsubstituted C6 to C40 arylaminegroup, provided that at least one of R³³ to R³⁸ is a substituted orunsubstituted C4 to C20 alkyl group.

If any of R³³, R³⁴, R³⁵, R³⁶, R³⁷, and R³⁸ is a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, or a substituted or unsubstituted C3 to C20heteroaryl group, it may be each independently fused with an adjacentaromatic to provide a C8 to C40 fused ring.

At least two of R³³ to R³⁶ may be an unsubstituted C4 to C10 linear orbranched alkyl group.

The first hole transporting material may include at least one ofcompounds represented by Chemical Formula A to Chemical Formula F.

The first hole transporting material may have a solubility (e.g., of atleast 1 wt %) in a non-polar solvent, thereby forming a solution when itis dissolved in the non-polar solvent.

The first hole transporting material may be included in an amount ofgreater than or equal to 2 weight percent (wt %) and less than 50 wt %based on a total weight of the emission layer.

A root mean square roughness of the surface of the emission layermeasured by atomic-force microscopy (AFM) may be about 0.5 nm to about2.0 nm.

The plurality of quantum dots may further include a hydrophobic(organic) ligand bound to a surface thereof.

The emission layer (or the quantum dots) may not include cadmium. Thequantum dots may include a core including a first semiconductornanocrystal, and a shell disposed on the core and including a secondsemiconductor nanocrystal.

The plurality of quantum dots (or the first or the second semiconductornanocrystal) may include each independently a Group II-VI compound, aGroup III-V compound, a Group IV-VI compound, a Group IV element orcompound, a Group I-III-VI compound, a Group I-II-IV-VI compound, or acombination thereof.

The second hole transporting material and the first hole transportingmaterial may be a different material.

The second hole transporting material may include apoly(3,4-ethylenedioxythiophene) or derivative, a poly(styrenesulfonate) or derivative, a poly(N-vinylcarbazole) or derivative, apoly(phenylene vinylene) or derivative, a poly((C1 to C6alkyl)(meth)acrylate) or derivative, a poly(C6 to C40 arylamine) orderivative, a polyaniline or derivative, a polypyrrole or derivative, apoly(9,9-dioctylfluorene) or derivative, a poly(spiro-fluorene) orderivative, apoly-(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine)or derivative, NiO, MoO₃, or a combination thereof.

The electron transport layer may include an inorganic nanoparticle, aquinolone compound, a triazine compound, a quinoline compound, atriazole compound, a naphthalene compound, or a combination thereof.

The electron transport layer may include a plurality of inorganicnanoparticles. The inorganic nanoparticle may include a compoundrepresented by Chemical Formula A:Zn_(1-x)M_(x)O  Chemical Formula A

wherein, in Chemical Formula 1,

M is Mg, Ca, Zr, W, Li, Ti, Y, Al, or a combination thereof, and

0≤x≤0.5.

The electroluminescent device may have a maximum external quantumefficiency (EQE) of greater than or equal to about 10%. The device mayhave a maximum luminance of greater than or equal to about 10,000candela per square meter (cd/m²). The electroluminescent device may havea peak capacitance of greater than or equal to about 0.8 nanofarads(nF), greater than or equal to about 1 nF, greater than or equal toabout 8 nF, and less than or equal to about 50 nF. Theelectroluminescent device may have T90 (or T95) of greater than or equalto about 20 hours, when it is operated at 1,500 candela per square meter(cd/m², also referred to herein as “nit”).

The electron transport layer may have an average thickness of about 20nanometers (nm) to about 100 nm.

A hole injection layer may be further disposed between the firstelectrode and the hole transport layer.

According to another embodiment, a display device including theelectroluminescent device is provided.

The electroluminescent device having improved device characteristics dueto improved hole transporting capability and surface characteristics ofthe emission layer and a display device including the same are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects will become apparent and more readilyappreciated from the following description of the embodiments, taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice according to an embodiment,

FIGS. 2 to 5 are atomic force microscopic (AFM) images showing surfacemorphology of the emission layer thin films (Comparative PreparationExample 1 (FIG. 2), Preparation Example 3 (FIG. 3), ComparativePreparation Example 2 (FIG. 4), and Preparation Example 5 (FIG. 5)),

FIGS. 6 to 9 are scanning electron microscopic images of the emissionlayer thin films (Comparative Preparation Example 3 (FIG. 6),Comparative Preparation Example 4 (FIG. 7), Preparation Example 10 (FIG.8), and Preparation Example 11 (FIG. 9)),

FIG. 10 is a graph of current density (milliamperes per squarecentimeter, mA/cm²) versus voltage (volts, V) showing voltage-currentdensity (log scale) characteristics of the electroluminescent devicesaccording to Examples 1 to 4 and Comparative Example 1,

FIG. 11 is a graph of current density (mA/cm²) versus voltage (V)showing voltage-current density (log scale) characteristics of theelectroluminescent devices according to Examples 9 to 12 and ComparativeExample 2,

FIG. 12 is a graph of luminance (%) versus time (hours) showinglife-span characteristics of the electroluminescent devices according toExamples 2, 5, 11, and 14 and Comparative Examples 1 and 2, and

FIG. 13 is a graph of luminance (%) versus time (hours) showinglife-span characteristics of the electroluminescent devices according toExamples 3, 6, 7, and 8 and Comparative Examples 1 and 2.

FIG. 14 is a graph of fluorescence intensity (nominal units) versus time(nanoseconds, ns) and shows results of time resolved photoluminescencespectroscopy analysis with respect to the emission at a wavelength of420 nm (TPD probe) for the films of the compositions of PreparationExample 12 and Comparative Preparation Example 5.

FIG. 15 is a graph of fluorescence intensity (nominal units) versus time(nanoseconds, ns) shows results of time resolved photoluminescencespectroscopy analysis with respect to the emission at a wavelength of630 nm (QD probe) for the films of the compositions of PreparationExample 12 and Comparative Preparation Example 5.

FIG. 16 is a graph of emission intensity (arbitrary units, a.u.) versusexcitation wavelength (nanometers, nm) and shows results of analyzingexcitation wavelength dependent photoluminescence emission behaviors forthe films prepared from the compositions of Preparation Example 12 andComparative Preparation Example 5.

DETAILED DESCRIPTION

Example embodiments of the present disclosure will hereinafter bedescribed in detail, and may be easily performed by a person having anordinary skill in the related art. However, this disclosure may beembodied in many different forms, and is not to be construed as limitedto the example embodiments set forth herein.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms, including “at least one,” unless the contentclearly indicates otherwise. “Combinations” is inclusive of blends,mixtures, alloys, reaction products, and the like. “Or” means “and/or”unless clearly indicated otherwise by context. Reference throughout thespecification to “an embodiment” means that a particular elementdescribed in connection with the embodiment is included in at least oneembodiment described herein, and may or may not be present in otherembodiments. The described elements may be combined in any suitablemanner in the various embodiments. “Combination thereof” is an open termthat includes one or more of the named elements, optionally togetherwith a like element not named.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, “a first element,” “component,” “region,” “layer” or“section” discussed below could be termed a second element, component,region, layer or section without departing from the teachings herein.

The endpoints of all ranges directed to the same component or propertyare inclusive and independently combinable (e.g., ranges of “less thanor equal to 25 wt %, or 5 wt % to 20 wt %,” is inclusive of theendpoints and all intermediate values of the ranges of “5 wt % to 25 wt%,” etc.). Disclosure of a narrower range or more specific group inaddition to a broader range is not a disclaimer of the broader range orlarger group. “About” or “approximately” as used herein is inclusive ofthe stated value and means within an acceptable range of deviation forthe particular value as determined by one of ordinary skill in the art,considering the measurement in question and the error associated withmeasurement of the particular quantity (i.e., the limitations of themeasurement system). For example, “about” can mean within one or morestandard deviations, or within +30%, 20%, 10%, 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this general inventive conceptbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand the present disclosure, and will not be interpreted in an idealizedor overly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. In the drawings, the thickness of layers, films, panels,regions, etc., are exaggerated for clarity. As such, variations from theshapes of the illustrations as a result, for example, of manufacturingtechniques and/or tolerances, are to be expected. Thus, embodimentsdescribed herein should not be construed as limited to the particularshapes of regions as illustrated herein but are to include deviations inshapes that result, for example, from manufacturing. For example, aregion illustrated or described as flat may, typically, have roughand/or nonlinear features. Moreover, sharp angles that are illustratedmay be rounded. Thus, the regions illustrated in the figures areschematic in nature and their shapes are not intended to illustrate theprecise shape of a region and are not intended to limit the scope of thepresent claims. Like reference numerals designate like elementsthroughout the specification and drawings.

Spatially relative terms, such as “beneath,” “under,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. For example, if the device inthe figures is turned over, elements described as “below” or “beneath”other elements or features would then be oriented “above” the otherelements or features. Thus, the exemplary term “below” can encompassboth an orientation of above and below. The device may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein interpreted accordingly. It will beunderstood that when an element such as a layer, film, region, orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.

As used herein, the term “alkyl group” refers to a group derived from astraight or branched chain saturated aliphatic hydrocarbon having thespecified number of carbon atoms and having a valence of at least one.Specific examples of the alkyl group may be a methyl group, an ethylgroup, an isopropyl group, a tert-butyl group, an n-octyl group, ann-decyl group, an n-hexadecyl group, and the like.

As used herein, the term “alkenyl group” refers to a straight orbranched chain, monovalent hydrocarbon group having at least onecarbon-carbon double bond. Specific examples of the alkenyl group may bea vinyl group, an allyl group, a 2-butenyl group, a 3-pentenyl group,and the like.

As used herein, the term “alkynyl group” refers to a straight orbranched chain, monovalent hydrocarbon group having at least onecarbon-carbon triple bond. Specific examples of the alkynyl group may bea propargyl group, a 3-pentynyl group, and the like.

As used herein, the term “alkoxy group” refers to “alkyl-O—”, whereinthe term “alkyl” has the same meaning as described above.

As used herein, the term “cycloalkyl group” refers to a monovalent grouphaving one or more saturated rings in which all ring members are carbon.

As used herein, the term “aryl group” refers to an aromatic hydrocarboncontaining at least one ring and having the specified number of carbonatoms. The term “aryl” may be construed as including a group with anaromatic ring fused to at least one cycloalkyl ring.

As used herein, the term “arylalkylene group” refers to an alkylenegroup substituted with an aryl group.

As used herein, the term “heteroaryl group” refers to an aryl groupcontaining one to three heteroatoms selected from the group consistingof N, O, S, Si, and P as ring-forming elements, and optionallysubstituted with one or more substituents where indicated. Examples ofheteroaryl groups include, but are not limited to, pyridyl, indolyl,carbazolyl, pyrimidinyl, pyridizinyl, pyrazinyl, imidazolyl, oxazolyl,furanyl, thiophenyl, thiazolyl, triazolyl, tetrazolyl, isoxazolyl,quinolinyl, pyrrolyl, pyrazolyl, benz[b]thiophenyl, isoquinolinyl,quinazolinyl, quinoxalinyl, thienyl, isoindolyl, and5,6,7,8-tetrahydroisoquinoline.

As used herein, when a definition is not otherwise provided, the term“heteroarylalkylene” refers to an alkylene group substituted with aheteroaryl group.

As used herein, the terms “alkylene”, “cycloalkylene”, “arylene”, and“heteroarylene” refer to a divalent group respectively derived from analkyl group, a cycloalkyl group, an aryl group, and a heteroaryl groupas defined above.

As used herein, the term “alkylamine group” refers to —NRR′ wherein Rand R′ are each independently a C1 to C20 alkyl group. As used herein,the term “arylamine group” refers to —NRR′ wherein R and R′ are eachindependently a C6 to C30 aryl group.

As used herein, when a definition is not otherwise provided,“substituted” refers to a group or compound wherein at least one of thehydrogen atoms thereof is substituted with a halogen atom (F, Cl, Br, orI), a hydroxy group, an alkoxy group, a nitro group, a cyano group, anamino group, an azido group, an amidino group, a hydrazino group, ahydrazono group, a carbonyl group, a carbamyl group, a thiol group, anester group, a carboxylic acid group or a salt thereof, a sulfonic acidgroup or a salt thereof, a phosphoric acid group or a salt thereof, a C1to C20 alkyl group, a C2 to C20 alkenyl group, a C2 to C20 alkynylgroup, a C6 to C30 aryl group, a C7 to C30 arylalkyl group, a C1 to C30alkoxy group, a C1 to C20 heteroalkyl group, a C3 to C20 heteroarylalkylgroup, a C3 to C30 cycloalkyl group, a C3 to C15 cycloalkenyl group, aC6 to C15 cycloalkynyl group, a C3 to C30 heterocycloalkyl group, or acombination thereof. “Substituted” is intended to be non-limiting andinclude inorganic substituents or organic substituents.

A group or group may also be referred to herein as “unsubstituted” or byequivalent terms such as “non-substituted,” which refers to the originalgroup in which a non-hydrogen moiety does not replace a hydrogen withinthat group.

As used herein, when a definition is not otherwise provided, the term“hetero” refers to a compound or group including one to threeheteroatoms that are N, O, S, P, and/or Si.

As used herein, “Group” refers to a group of Periodic Table of theElements.

As used herein, “Group II” refers to Group IIA and Group IIB, andexamples of Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, examples of “Group II metal that does not include Cd”refers to a Group II metal except Cd, for example Zn, Hg, Mg, etc.

As used herein, “Group III” refers to Group IIIA and Group IIIB, andexamples of Group III metal can be Al, In, Ga, and Tl, but are notlimited thereto.

As used herein, “Group IV” refers to Group IVA and Group IVB, andexamples of a Group IV metal can be Si, Ge, and Sn, but are not limitedthereto. As used herein, the term “metal” can include a semi-metal suchas Si.

As used herein, “Group I” refers to Group IA and Group IB, and examplesinclude Li, Na, K, Rb, and Cs, but are not limited thereto.

As used herein, “Group V” refers to Group VA, and examples includenitrogen, phosphorus, arsenic, antimony, and bismuth, but are notlimited thereto.

As used herein, “Group VI” refers to Group VIA, and examples includesulfur, selenium, and tellurium, but are not limited thereto.

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice 10 according to an embodiment.

According to an embodiment, an electroluminescent device 10 includes afirst electrode 110 and a second electrode 160 facing each other and anemission layer 140 disposed therebetween and including a plurality ofquantum dots 141 and a first hole transporting material. The first holetransporting material may include an organic material having asubstituted or unsubstituted C4 to C20 alkyl group attached to abackbone structure. A hole transport layer 130 including a second holetransporting material may be further disposed between the firstelectrode 110 and the emission layer 140. If desired, a hole injectionlayer 120 may be further disposed between the first electrode 110 andthe hole transport layer 130. In addition, an electron transport layer150 may be further disposed between the emission layer 140 and thesecond electrode 160.

In an embodiment, the electroluminescent device 10 has a stack structurewherein the hole injection layer 120, the hole transport layer 130, theemission layer 140 and the electron transport layer 150 are disposed inthis order between the first electrode 110 and the second electrode 160facing each other.

In an embodiment, the first electrode 110 may be directly connected to adriving power source so may function to flow current to the emissionlayer 140. The first electrode 110 may include a material having lighttransmittance in at least a visible light wavelength region, but is notlimited thereto. The first electrode 110 may include a material havinglight transmittance in an infrared or ultraviolet (UV) wavelengthregion.

For example, the first electrode 110 may be an optically transparentmaterial.

In an embodiment, the first electrode 110 may include molybdenum oxide,tungsten oxide, vanadium oxide, rhenium oxide, niobium oxide, tantalumoxide, titanium oxide, zinc oxide, nickel oxide, copper oxide, cobaltoxide, manganese oxide, chromium oxide, indium oxide, or a combinationthereof.

However, the first electrode 110 according to an embodiment is notnecessarily limited thereto but may include a material further havinglight transmittance with respect to light in an infrared or ultraviolet(UV) wavelength region or a semi-permeable material selectivelytransmitting light in a particular wavelength region and conduct afunction of reflecting light in a visible light wavelength region andturning it back toward the second electrode 160.

Meanwhile, in an embodiment, the first electrode 110 may be disposed onthe substrate 100 as shown in FIG. 1. The substrate 100 may be atransparent insulating substrate or may be made of a ductile material.The substrate 100 may include glass or a polymer material in a filmhaving a glass transition temperature (Tg) of greater than about 150° C.For example, it includes a COC (cycloolefin copolymer) or COP(cycloolefin polymer) based material.

In an embodiment, the substrate 100 may support the hole injection layer120, the hole transport layer 130, the emission layer 140, and theelectron transport layer 150 disposed between the first electrode 110and the second electrode 160. However, the substrate 100 of theelectroluminescent device 10 according to an embodiment may not bedisposed under the first electrode 110, but the substrate 100 may bedisposed on the second electrode 160 or may be omitted, as needed.

The second electrode 160 includes an optically transparent material andmay function as a light-transmitting electrode to transmit lightgenerated in the emission layer 140. In an embodiment, the secondelectrode 160 may include at least one of silver (Ag), aluminum (Al),copper (Cu), gold (Au), and an alloy thereof, molybdenum oxide, tungstenoxide, vanadium oxide, rhenium oxide, niobium oxide, tantalum oxide,titanium oxide, zinc oxide, nickel oxide, copper oxide, cobalt oxide,manganese oxide, chromium oxide, indium oxide, or a combination thereof.

However, the second electrode 160 according to an embodiment is notnecessarily limited thereto, and may include a semi-permeable materialselectively transmitting light in a particular wavelength region, andreflecting a portion of light in another visible light wavelength regionback towards the first electrode 110.

When the second electrode 160 functions as a reflecting electrode, thefirst electrode 110 may be a light-transmitting electrode formed of amaterial capable of transmitting light in at least a visible lightwavelength region or a semi-permeable electrode selectively transmittinglight in a particular wavelength region.

Each of the first electrode 110 and the second electrode 160 may beformed by depositing a material for forming an electrode on thesubstrate 100 or an organic layer by a method such as sputtering.

On the other hand, as shown in FIG. 1, an electroluminescent device 10according to an embodiment may have a conventional structure wherein thesubstrate 100 and each of constituent elements are disposed in theabove-described stacking order.

However, the electroluminescent device 10 according to an embodiment isnot necessarily limited thereto and may have various structures within arange of satisfying the aforementioned order of disposing eachconstituent element. For example, when the substrate 100 is disposed notbeneath the first electrode 110 but on the second electrode 160, theelectroluminescent device 10 may have an inverted structure.

The hole injection layer 120 may be disposed on, for example directlyon, the first electrode 110. The hole injection layer 120 may supplyholes into the emission layer 140 through the hole transport layer 130.However, the hole injection layer 120 is optional and may be omittedconsidering the thickness and the material of the hole transport layer130.

The hole injection layer 120 may be formed of a p-type semiconductormaterial or a material doped with a p-type dopant. For example, the holeinjection layer 120 may include a [poly(3,4-ethylenedioxythiophene)](PEDOT) or derivative, a [poly(styrene sulfonate)] (PSS) or derivative,a poly(N-vinylcarbazole) (PVK) or derivative, a poly(phenylenevinylene)or derivative such as a poly(p-phenylenevinylene) (PPV) derivative, apoly((C1 to C6 alkyl) (meth)acrylate) or derivative, apoly(9,9-dioctylfluorene) or derivative, a poly(spiro-fluorene) orderivative, tris(4-carbazol-9-yl-phenyl)amine (TCTA),N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD), N,N′-di(naphthalen-1-yl)-N—N′-diphenyl-benzidine (NPB),tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA),poly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamine](TFB),poly(9,9-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-1,4-diaminobenzene)(PFB), poly-TPD, a metal oxide such as NiO and MoO₃, or a combinationthereof, but is not limited thereto.

The hole transport layer 130 may be disposed on the first electrode 110,for example on the first electrode 110 and the hole injection layer 120.The hole transport layer 130 may provide and transport holes into theemission layer 140. The hole transport layer 130 may be formed directlyunder the emission layer 140 and directly contacts the emission layer140.

In an embodiment, the hole transport layer 130 includes a second holetransporting material. The second hole transporting material may be ap-type semiconductor material, or a material doped with a p-type dopant.

In an embodiment, the second hole transporting material included in thehole transport layer 130 may be a different material from the first holetransporting material of the emission layer 140 that will be describedlater. In an embodiment, the second hole transporting material may be apolymer, an oligomer, a metal oxide, or a combination thereof.

Examples of the second hole transporting material may be apoly(3,4-ethylenedioxythiophene) or derivative, a poly(styrenesulfonate) or derivative, a poly-N-vinylcarbazole or derivative, apoly(phenylene vinylene) or derivative such as a poly(para-phenylenevinylene), a poly(C1 to C6 alkyl)(meth)acrylate) or derivative, apoly(C6 to C40 arylamine) or derivative, a polyaniline or derivative, apolypyrrole or derivative, a poly(9,9-dioctylfluorene) or derivative,for example apoly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamine]or apoly((9,9-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-1,4-diaminobenzene),a poly(spiro-fluorene) derivative, apoly-(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine)or derivative, NiO, MoO₃, or a combination thereof, but are not limitedthereof.

When the hole transport layer 130 is made of a polymer, an oligomer, ametal oxide, or a combination thereof as the second hole transportingmaterial, life-span of the light emitting device may be increased, andturn-on voltages of the electroluminescent device 10 that are a workinginitiation voltages may be lowered. Particularly, when the second holetransporting material is selected from the above-mentioned materials,surface morphology of the hole transport layer 130 in direct contactwith the emission layer 140 may be uniformly controlled compared withwhen the monomolecular hole transporting material is used. Accordingly,leakage paths of the holes moving from the hole transport layer 130 tothe emission layer 140 can be minimized, so that leakage currents anddriving voltages may be lowered.

For example, the hole transport layer 130 may be formed in a wet coatingmethod such as spin coating and the like. For example, both of the holetransport layer 130 and the emission layer 140 may be formed in a wetcoating method. In this way, the hole transport layer 130 and/or theemission layer 140 may be formed in a simple process.

In addition, in an embodiment, the hole transport layer 130 and theemission layer 140 may be made of materials having relatively differentsolubilities. For example, the hole transport layer 130 has improvedsolubility in an aromatic non-polar solvent, and the emission layer 140has improved solubility in an aliphatic non-polar solvent. When the holetransport layer 130 is dissolved in the aromatic non-polar solvent, theemission layer 140 is dissolved in the aliphatic non-polar solvent, andthen a solution process is performed, the emission layer 140 may beformed without surface damage of the previously formed hole transportlayer 130.

For example, when a TFB polymer film is formed as the hole transportlayer 130, a precursor solution including a TFB precursor polymer and anaromatic non-polar solvent (e.g., toluene, xylene, etc.) is spin-coatedon the first electrode 110 or the hole injection layer 120, thermaltreatment is performed in an inert gas atmosphere of N₂ or in a vacuumat a temperature of about 150° C. to about 180° C. for about 30 minutesto form a hole transport layer 130 made of TFB, and the emission layer140 may be conveniently formed thereon using an aliphatic non-polarsolvent (for example, octane, nonane, cyclohexane, or the like) using asolution process.

In this way, when the solvent selectivity of the hole transport layer130 and the emission layer 140 is relatively different from each other,both of the hole transport layer 130 and the emission layer 140 may beformed using a solution process, and thus it is possible to perform theprocess conveniently and to minimize the damage of the surface of thehole transport layer 130 by the organic solvent or the like in theprocess of forming the subsequent emission layer 140 by differentsolvent selectivity.

The emission layer 140 may be disposed between the first and the secondelectrodes. The emission layer may be disposed on the hole transportlayer 130. The emission layer may include the plurality of quantum dots141 and the first hole transporting material.

The emission layer 140 (or the quantum dots) may be a site whereelectrons and holes transported by a current supplied from the firstelectrode 110 and the second electrode 160 are combined to generateexcitons, and the generated excitons are transited from an exited stateto a ground state to emit light in a wavelength corresponding to thesize and/or the composition of the quantum dots 141. That is, thequantum dots 141 may endow the emission layer 140 with anelectro-luminescence function. The quantum dots are configured to emit afirst light upon excitation by light.

The first light may be green light, red light, or blue light. The firstlight has a first maximum peak wavelength. The first maximum peakwavelength may be present in a range of a wavelength region that will bedescribed below.

The quantum dots 141 have a discontinuous energy bandgap by the quantumconfinement effect and incident light may be converted into light havinga particular wavelength and then radiated by emission of light.Accordingly, the emission layer 140 including the quantum dots 141 mayproduce light having excellent color reproducibility and color purity.

The emission layer 140 may emit light in a predetermined wavelengthregion. In an embodiment, the emission layer may be configured to emitthe same light as the first light when a voltage is applied thereon. Thepredetermined wavelength region can belong to a visible light region.The predetermined wavelength region or the first peak wavelength of thequantum dots may be in any of a first wavelength region of about 380 nmto about 488 nm, a second wavelength region of about 490 nm to about 510nm, a third wavelength region of greater than 510 nm to equal to or lessthan 580 nm, a fourth wavelength region of about 582 nm to about 600 nm,and a fifth wavelength region of about 620 nm to about 680 nm. In anembodiment, the quantum dots may emit green light and the predeterminedwavelength region or the first peak wavelength of the quantum dots maybe present in a range of greater than or equal to 500 nm, greater thanor equal to about 510 nm, or greater than or equal to about 520 nm andless than or equal to about 580 nm, less than or equal to about 570 nm,less than or equal to about 560 nm, less than or equal to about 550 nm,or less than or equal to about 540 nm. In an embodiment, the quantumdots may emit red light and the predetermined wavelength region or thefirst peak wavelength of the quantum dots may be present in a range ofgreater than or equal to 600 nm, greater than or equal to about 610 nm,greater than or equal to about 620 nm, or greater than or equal to about625 nm and less than or equal to about 680 nm, less than or equal toabout 670 nm, less than or equal to about 660 nm or greater than orequal to about 650 nm. In an embodiment, the quantum dots may emit bluelight and the predetermined wavelength region or the first peakwavelength of the quantum dots may be present in a range of greater thanor equal to 440 nm, greater than or equal to about 450 nm, greater thanor equal to about 460 nm, or greater than or equal to about 465 nm andless than or equal to about 490 nm, less than or equal to about 480 nm,less than or equal to about 475 nm, or less than or equal to about 470nm.

The emission layer may be configured to emit light on the voltageapplication without formation of triplet emitter. In an embodiment, theemission layer may not include a phosphorescence dopant such as anorganic metal compound. Examples of the organic metal compound for thephosphorescence dopant may include an iridium containing organic metalcompound, a platinum containing organic metal compound, or a combinationthereof.

The emission layer may not include cadmium, lead, or a combinationthereof.

In an embodiment, each of the quantum dots 141 according to anembodiment may include a Group II-VI compound, a Group III-V compound, aGroup IV-VI compound, a Group IV element or compound, a Group I-III-VIcompound, a Group I-II-IV-VI compound, or a combination thereof.

The quantum dots may have a core shell structure having a core includinga first semiconductor nanocrystal and a shell disposed on the core, theshell including a second semiconductor nanocrystal having a differentcomposition from the first semiconductor nanocrystal. The shell mayinclude a metal chalcogenide. The quantum dots may include zinc andsulfur in the shell or an outermost layer of the shell.

The first semiconductor nanocrystal and the second semiconductornanocrystal may include each independently a Group II-VI compound, aGroup III-V compound, a Group IV-VI compound, a Group IV element orcompound, a Group I-III-VI compound, a Group I-II-IV-VI compound, or acombination thereof. The first semiconductor nanocrystal may includezinc, indium, or a combination thereof. The quantum dots (or the firstsemiconductor nanocrystal) may include InP, InZnP, ZnSe, ZnSeTe, or acombination thereof. The quantum dots may include zinc.

The Group II-VI compound may be a binary element compound such as ZnS,ZnSe, ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combination thereof; aternary element compound such as ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe,HgSTe, HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combination thereof;and a quaternary element compound such as HgZnTeS, HgZnSeS, HgZnSeTe,HgZnSTe, or a combination thereof. The Group II-VI compound may furtherinclude a Group III metal.

The Group III-V compound may be a binary element compound such as GaN,GaP, GaAs, GaSb, AlN, AlP, AlAs, AlSb, InN, InP, InAs, InSb, or acombination thereof; a ternary element compound such as GaNP, GaNAs,GaNSb, GaPAs, GaPSb, AlNP, AlNAs, AlNSb, AlPAs, AlPSb, InNP, InNAs,InNSb, InPAs, InPSb, InZnP, or a combination thereof; and a quaternaryelement compound such as GaAlNP, GaAlNAs, GaAlNSb, GaAlPAs, GaAlPSb,GaInNP, GaInNAs, GaInNSb, GaInPAs, GaInPSb, InAlNP, InAlNAs, InAlNSb,InAlPAs, InAlPSb, or a combination thereof. The Group III-V compound mayfurther include a Group II metal (e.g., InZnP).

The Group IV-VI compound may be a binary element compound such AsSnS,SnSe, SnTe, PbS, PbSe, PbTe, or a combination thereof; a ternary elementcompound such as SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS,SnPbSe, SnPbTe, or a combination thereof; and a quaternary elementcompound such as SnPbSSe, SnPbSeTe, SnPbSTe, or a combination thereof.Examples of the Group I-III-VI compound may be CuInSe₂, CuInS₂,CuInGaSe, and CuInGaS, are not limited thereto.

Examples of the Group I-II-IV-VI compound may be CuZnSnSe and CuZnSnS,but are not limited thereto. Examples of the Group IV compound may be asingle substance such as Si, Ge, or a combination thereof; and a binaryelement compound such as SiC, SiGe, or a combination thereof.

The binary element compound, the ternary element compound, or thequaternary element compound respectively can exist in a uniformconcentration in the particle or in partially different concentrationsin the same particle.

According to an embodiment, the quantum dots may have a core-shellstructure including one semiconductor nanocrystal core particle andanother semiconductor nanocrystal shell surrounding the core. The coreand the shell may have a concentration gradient wherein theconcentration of the element(s) of the shell decreases or increases in adirection towards the core. In addition, the quantum dots may have onesemiconductor nanocrystal core and multiple shells surrounding the core.

Herein, the multi-layered shell structure has a structure of two or moreshells and each layer may have a single composition or an alloy and/ormay have a concentration gradient.

When the quantum dot has a core-shell structure, a material compositionof the shell has a larger energy bandgap than that of the core, whichmay exhibit an effective quantum confinement effect. However, theembodiment is not limited thereto.

Meanwhile, in the multi-layered shell, a shell that is outside of thecore may have a higher energy bandgap than a shell that is nearer to thecore and quantum dots may have an ultraviolet (UV) to infraredwavelength emission ranges.

The quantum dots may have quantum efficiency of greater than or equal toabout 10%, for example, greater than or equal to about 20%, greater thanor equal to about 30%, greater than or equal to about 40%, greater thanor equal to about 50%, greater than or equal to about 60%, greater thanor equal to about 70%, greater than or equal to about 90%, about 100%,or even 100%.

In a display device, the quantum dots may have a relatively narrowspectrum so as to improve color purity or color reproducibility. Thequantum dots may have for example a full width at half maximum (FWHM) ata photoluminescence wavelength spectrum of less than or equal to about45 nm, less than or equal to about 40 nm, or less than or equal to orabout 30 nm. Within the ranges, color purity or color reproducibility ofa display device may be improved.

The quantum dots may have an average particle diameter (a diameter or anequivalent diameter measured/calculated from a (e.g., two dimensional)image obtained by an electron microscope) of about 1 nm to about 100 nm.For example, the quantum dots may have an average particle diameter ofabout 1 nm to about 20 nm, for example, about 2 nm (or about 3 nm) toabout 20 nm (or about 15 nm, or about 10 nm, or about 9 nm).

In addition, the shapes of the quantum dots may be general shapes inthis art and thus may not be particularly limited. For example, thequantum dots may have a spherical, oval, tetrahedral, pyramidal,cuboctahedral, cylindrical, polyhedral, multi-armed, or cubenanoparticle, nanotube, nanowire, nanofiber, nanosheet, or a combinationthereof. The quantum dots may have any cross-sectional shape.

The quantum dots may be commercially available or may be synthesized inany method. For example, several nano-sized quantum dots may besynthesized according to a wet chemical process. In the wet chemicalprocess, precursors react in an organic solvent to grow crystalparticles, and the organic solvent or a ligand compound may coordinatethe surface of the quantum dot, controlling the growth of the crystal.Examples of the organic solvent and the ligand compound are known.

An excess amount of an organic material that is not coordinated on(bound to) the surface of the nanocrystals may be removed by pouring itin excessive non-solvent, and centrifuging the resulting mixture.Examples of the non-solvent may be acetone, ethanol, methanol, and thelike, but are not limited thereto. The quantum dots may include anorganic material (e.g., (organic) ligand compound, an organic solvent,or a combination thereof) in an amount of less than or equal to about50% by weight, for example, less than or equal to about 30 wt %, lessthan or equal to about 20 wt %, or less than or equal to about 10 wt %based on a weight of the quantum dots. The organic material may includea ligand compound, an organic solvent, or a combination thereof.

The electroluminescent device 10 according to an embodiment may furtherinclude a ligand (hereinafter, referred to as a hydrophobic ligand)having a hydrophobic moiety bound to the surface of the quantum dots141. In an embodiment, the hydrophobic ligand may include a functionalgroup bound to the surface of the quantum dots 141 and a hydrophobicfunctional group providing hydrophobicity.

The hydrophobic moiety may be for example a C4 to C20 alkyl group, a C4to C20 alkenyl group, a C4 to C20 alkynyl group, or a combinationthereof and the functional group bound to the surface of the quantumdots 141 may be for example a hydroxy group (—OH), a carboxyl group(—COOH), an amine group, a thiol group, a phosphine group, a phosphonicacid group, a phosphonic acid group, or the like.

Examples of the hydrophobic ligand may be fatty acid such as oleic acid,stearic acid, palmitic acid, and the like. The organic ligand mayinclude RCOOH, RNH₂, R₂NH, R₃N, RSH, R₃PO, R₃P, ROH, RCOOR, RPO(OH)₂,RHPOOH, R₂POOH, or a combination thereof (wherein, R is eachindependently a C3 (or C5) to C40 substituted or unsubstituted aliphatichydrocarbon group such as C3 to C40 alkyl, alkenyl, or alkynyl or thelike, a C6 to C40 substituted or unsubstituted aromatic hydrocarbongroup such as C6 to C40 aryl group, or a combination thereof).

Examples of the organic ligand may include a thiol compound such asmethane thiol, ethane thiol, propane thiol, butane thiol, pentane thiol,hexane thiol, octane thiol, dodecane thiol, hexadecane thiol, octadecanethiol, or benzyl thiol; an amine compounds such as methylamine,ethylamine, propylamine, butane amine, pentylamine, hexylamine,octylamine, nonylamine, decylamine, dodecylamine, hexadecylamine,octadecylamine, dimethylamine, diethylamine, dipropylamine,tributylamine, or trioctylamine; a carboxylic acid compound such asmethanoic acid, ethanoic acid, propanoic acid, butanoic acid, pentanoicacid, hexanoic acid, heptanoic acid, octanoic acid, dodecanoic acid,hexadecanoic acid, octadecanoic acid, oleic acid, or benzoic acid; aphosphine compound such as methyl phosphine, ethyl phosphine, propylphosphine, butyl phosphine, pentyl phosphine, octyl phosphine, dioctylphosphine, tributyl phosphine, or trioctyl phosphine; an oxide of aphosphine compound such methyl phosphine oxide, ethyl phosphine oxide,propyl phosphine oxide, butyl phosphine oxide, pentyl phosphine oxide,tributyl phosphine oxide, octyl phosphine oxide, dioctyl phosphineoxide, or trioctyl phosphine oxide; a diphenyl or triphenyl phosphinecompound, or an oxide compound thereof; a C5 to C20 alkyl phosphonicacid; a C5 to C20 alkyl phosphinic acid, such as hexyl phosphinic acid,octyl phosphinic acid, dodecane phosphinic acid, tetradecane phosphinicacid, hexadecane phosphinic acid, octadecane phosphinic acid; and thelike, but are not limited thereto.

When the hydrophobic (organic) ligand is included as described, thequantum dots 141 may exhibit non-polarity (e.g., as a whole), and theemission layer 140 including the same may entirely exhibit non-polarity.The quantum dots 141 having the attached hydrophobic ligand may havesolvent selectivity for a non-polar solvent, specifically an aliphaticnon-polar solvent.

In an electroluminescent device 10 according to an embodiment, theemission layer 140 may include the quantum dots 141 in a small amount inorder to exhibit excellent luminous efficiency.

The quantum dots 141 may be included in an amount of for example greaterthan or equal to about 5 wt %, greater than or equal to about 10 wt %,greater than or equal to about 15 wt %, or greater than or equal toabout 20 wt %, greater than or equal to about 25 wt %, greater than orequal to about 30 wt %, greater than or equal to about 35 wt %, greaterthan or equal to about 40 wt %, greater than or equal to about 45 wt %,greater than or equal to about 50 wt %, greater than or equal to about55 wt %, greater than or equal to about 60 wt %, greater than or equalto about 65 wt %, greater than or equal to about 70 wt %, greater thanor equal to about 75 wt %, greater than or equal to about 80 wt %,greater than or equal to about 85 wt %, or greater than or equal toabout 95 wt % and less than or equal to about 98 wt %, less than orequal to about 95 wt %, less than or equal to about 90 wt %, less thanor equal to about 85 wt %, less than or equal to about 80 wt %, lessthan or equal to about 75 wt %, less than or equal to about 70 wt %,less than or equal to about 65 wt %, less than or equal to about 60 wt%, less than or equal to about 55 wt %, or less than or equal to about50 wt %, based on 100 wt % of the emission layer, but is not limitedthereto. In an embodiment, an amount of the quantum dots may be about 5wt % to about 98 wt %, about 20 wt % to about 98 wt %, about 20 wt % toabout 90 wt %, about 20 wt % to about 85 wt %, about 50 wt % to about 85wt % based on 100 wt % of the emission layer.

The amount of the quantum dots 141 may be adjusted considering materialsof the used quantum dots 141, a type of emitted light, an amount of aused organic ligand (e.g. hydrophobic ligand), and thicknesses of thehole transport layer 130, the emission layer 140 and/or the electrontransport layer 150.

In a device of an embodiment, the emission layer may include the quantumdots and a first hole transporting material mixed therewith. The firsthole transporting material may improve a hole transporting capability ofthe emission layer 140. The first hole transporting material may be adifferent material from the second hole transporting material of thehole transport layer 130. The first hole transporting material may be ap-type semiconductor material, or a material doped with a p-type dopantand may be selected from monomolecular to low molecular weightmaterials, or a combination thereof unlike the second hole transportingmaterial.

A conventional, quantum dot based electroluminescent device may suffer aserious problem of deterioration of the emission layer. When thedeterioration of the emission layer occurs, brightness of the devicedecreases, and an operating voltage thereof increases over the operationtime. Without wishing to be bound by any theory, such deterioration mayresult from the following reasons:

In a quantum dot based emission layer, a hole mobility may be lower thanan electron mobility and thus a hole-electron recombination tends tooccur at or near a HTL (hole transport layer)/QD interface. Such aninterfacial recombination may have an adverse effect on a lifetime ofthe device. When the hole injection into the QD emission layer isdelayed (or not facilitated), the holes may be accumulated at the HTL/QDinterface and deterioration of the interface may occur. The excesselectrons injected into the QD emission layer may cause thedeterioration of the HTL interface and/or even may cross the HTL tocombine with the holes, forming an exciton that also causes additionaldeterioration of the HTL.

The deterioration of the HTL may cause an increase in an operationvoltage of the device and further impede the hole injection, resultingin a reduction of a device efficiency and a decrease of a deviceluminance.

In addition, inefficient movements of the charges inside the quantum dotemission layer may bring forth a charge accumulation, which may in turnan increase of an E-Field applied inside the quantum dot and thereby thequantum efficiency of the quantum dot may be temporarily decreased byStark effect.

In the electroluminescent device of an embodiment, a single molecularcompound (e.g., the first hole transporting material) showing arelatively high hole mobility is mixed with the quantum dots in theemission layer. Without wishing to be bound by any theory, theintroduction of the aforementioned emission layer may improve the holeinjection property into the emission layer, whereby the HTL/QDinterfacial deterioration may be suppressed and the charge accumulation,which may possibly occur in the emission layer otherwise, may be reducedand relieved. In addition, without wishing to be bound by any theory, itis believed that an energy transfer via exciton may occur between thefirst hole transporting layer and the quantum dots, which may bringforth additional improvement of a device performance.

Therefore, in the emission layer included in the device of anembodiment, the first hole transporting material is configured to emit asecond light on excitation by light. The second light may have a second(maximum) peak wavelength. The second peak wavelength may be less thanthe first peak wavelength of the quantum dots. The difference betweenthe second peak wavelength and the first peak wavelength may be at leastabout 10 nm, at least about 50 nm, at least about 100 nm, at least about130 nm, at least about 150 nm, at least about 160 nm, at least about 170nm, at least about 180 nm, at least about 190 nm, at least about 200 nm,at least about 210 nm, at least about 220 nm, at least about 230 nm, atleast about 240 nm, at least about 250 nm, at least about 260 nm, atleast about 270 nm, at least about 280 nm, or at least about 300 nm. Thedifference between the second peak wavelength and the first peakwavelength may be less than or equal to about 600 nm, less than or equalto about 500 nm, or less than or equal to about 400 nm.

In an embodiment, the first peak wavelength of the quantum dot is in arange of about 600 nm to about 650 nm (red emission), the second peakwavelength of the second light may be greater than or equal to about 380nm, or greater than or equal to about 420 nm and less than or equal toabout 590 nm, less than or equal to about 500 nm or less than or equalto about 490 nm.

In an embodiment, the first peak wavelength of the quantum dot is in arange of about 500 nm to about 560 nm (green emission), the second peakwavelength may be greater than or equal to about 300 nm, greater than orequal to about 330 nm, greater than or equal to about 380 nm, or greaterthan or equal to about 420 nm and less than or equal to about 490 nm,480 nm, less than or equal to about 470 nm, or less than or equal toabout 460 nm.

In an embodiment, the first peak wavelength of the quantum dot is in arange of about 440 nm to about 470 nm (blue emission), the second peakwavelength may be greater than or equal to about 300 nm, greater than orequal to about 330 nm, greater than or equal to about 380 nm, or greaterthan or equal to about 420 nm and less than about 470 nm, less than orequal to about 460 nm, less than or equal to about 450 nm, less than orequal to about 440 nm, less than or equal to about 430 nm, less than orequal to about 420 nm, less than or equal to about 410 nm, less than orequal to about 400 nm, less than or equal to about 390 nm, less than orequal to about 380 nm, less than or equal to about 370 nm, less than orequal to about 360 nm, or less than or equal to about 350 nm.

In an electroluminescent state of the device (e.g., upon the applicationof the voltage with no excitation light), the first hole transportingmaterial may not emit light of the second peak wavelength. In anembodiment, the emission layer may be configured to emit light withoutthe formation of a triplet emitter. Thus, the emission layer may emitlight having substantially the same color as the first light of thequantum dots.

The electroluminescent emission layer may include at least two emissionlayers that are optically isolated with each other and repeating (e.g.arranged in a predetermined pattern) such as a first repeating emissionlayer, a second repeating emission layer, or the like and each of therepeating emission layer may be configured to emit light of a desiredcolor.

The quantum dots in the emission layer (e.g., in an individual emissionlayer) may be configured to emit light of the same color. As usedherein, “emission of light of the same color” may refer to the casewhere a difference between the (maximum) peak wavelengths thereof may beless than or equal to about 30 nm, for example, less than or equal toabout 20 nm, less than or equal to about 15 nm, less than or equal toabout 10 nm, or less than or equal to about 5 nm, In the device of theembodiment, a full width at half maximum of the light emitted from theemission layer (or the electroluminescent peak thereof) may be less thanor equal to about 60 nm, less than or equal to about 50 nm, less than orequal to about 40 nm, less than or equal to about 35 nm, less than orequal to about 30 nm, or less than or equal to about 25 nm or less thanor equal to about 20 nm.

In an embodiment, the time resolved photoluminescence (TRPL) is used todetermine a temporal evolution and a corresponding radiative lifetimewith respect to an emitted light (e.g., the first light or the secondlight) having a predetermined wavelength.

An apparatus for the TRPL analysis is commercially available. Theluminescence data may be fit with a multi-exponential decay of the form:I(t)=Σ_(i=1) ^(N) a _(i) e ^(−t/τ) ^(i)convolved with the instrument response function, where t is time, I(t)is an intensity at the time t, t_(i) and a_(i) correspond to thelifetime and the fractional contribution of each component to the decay,respectively.

In an embodiment, the first hole transporting material may be well fitby a one-term exponential composed of a fast initial decay. In anembodiment, the quantum dots of the emission layer may be well fit by athree-term exponential composed of a fast initial decay (t1), followedby a moderate decay (t2), in addition to a longer lived component (t3),and the decay time at each component is denoted as t1, t2, and t3,respectively. An average decay time (t_(avg)) is a weighted average ofthe decay times of the components.

In the TRPL analysis, the emission layer including the aforementionedcombination of the quantum dots and the first hole transporting materialmay show a t1 for the first light that is less than or equal to about 15ns, less than or equal to about 14 ns, less than or equal to about 13ns, less than or equal to about 12 ns, less than or equal to about 11ns, less than or equal to about 10 ns, or less than or equal to about 9ns. For example, the t1 for the first light may be in a range of fromabout 1 ns to about 15 ns, from about 2 ns to about 14 ns, from about 3ns to about 13 ns, from about 4 ns to about 12 ns, from about 5 ns toabout 11 ns, from about 6 ns to about 10 ns, from about 7 ns to about 9ns, or a combination thereof.

In the TRPL analysis of the emission layer, a decay fraction of the t1for the first light may be greater than or equal to about 5%, greaterthan or equal to about 10%, greater than or equal to about 15%, greaterthan or equal to about 20%, or greater than or equal to about 25%. Forexample, the decay fraction of the t1 for the first light may be in arange of from about 5% to about 40%, from about 10% to about 35%, fromabout 15% to about 30%, from about 20% to about 25%, or a combinationthereof.

In the case of a emission layer that includes the quantum dot but doesnot include the foregoing first hole transporting material, at the TRPLanalysis for the first light, the decay component (fraction) for the t1may not present (e.g., may be zero %) or if present may be less than orequal to about 5%, or less than about 1%. However, when the emissionlayer includes the foregoing hole transporting material together withthe quantum dots, the quantum dots included in the emission layer mayexhibit the fraction of the fast initial decay (t1), which indicatesthat a decay path for the exciton formed in the emission layer may bechanged.

In the TRPL analysis of the emission layer, an average decay time(t_(avg)) for the first light of the quantum dots may be less than orequal to about 30 ns, less than or equal to about 29 ns, less than orequal to about 28 ns, less than or equal to about 27 ns, or less than orequal to about 26 ns. The average decay time (t_(avg)) for the firstlight of the quantum dots may be greater than or equal to about 10 ns,greater than or equal to about 15 ns, or greater than or equal to about20 ns.

In the TRPL analysis of the emission layer, an average decay time(t_(avg)) for the second light of the first hole transporting materialmay be less than or equal to about 0.7 ns, less than or equal to about0.6 ns, less than or equal to about 0.5 ns, less than or equal to about0.4 ns, or less than or equal to about 0.3 ns. In the TRPL analysis ofthe emission layer, an average decay time (t_(avg)) for the second lightof the first hole transporting material may be greater than or equal toabout 0.1 ns, greater than or equal to about 0.2 ns, greater than orequal to about 0.3 ns, greater than or equal to about 0.4 ns, or greaterthan or equal to about 0.5 ns.

The emission layer of the embodiment includes the quantum dots togetherwith the first hole transporting material and an excitation wavelengthdependent photoluminescent behavior thereof includes not only theemission of the quantum dots but also the emission with which thequantum dots and the hole transporting material are related together andthus is different from a emission layer including only the quantum dots.

In an excitation wavelength dependent photoluminescence emissionbehavior analysis (hereinafter, a PLE analysis), a light emittingintensity of the emission layer may be less than or equal to about 0.4,less than or equal to about 0.35, less than or equal to about 0.2, orless than or equal to about 0.15 with respect to a maximum intensitywhen it is excited with light of a wavelength of the second peakwavelength.

In the PLE analysis, a light emitting intensity of the emission layermay be less than or equal to about 0.4, less than or equal to about 0.3,less than or equal to about 0.2, less than or equal to about 0.15, orless than or equal to about 0.1 with respect to its maximum intensitywhen it is excited with light of a wavelength of 450 nm.

In an embodiment, the first hole transporting material may have asubstituted or unsubstituted C4 to C20 alkyl group attached to abackbone structure. The first hole transporting material may have anenhanced solubility with respect to a non-polar solvent, specifically analiphatic non-polar solvent.

In an embodiment, both of the first hole transporting material and thehydrophobic ligand may have solubility in an aliphatic non-polarsolvent. Accordingly, when the emission layer 140 is formed using thesolution, the formed emission layer 140 may have improved surfacemorphology and may have excellent hole transporting capability due tothe first hole transporting material.

As described, the emission layer 140 and the hole transport layer 130have solubility in an aliphatic non-polar solvent or an aromaticnon-polar solvent, and thus damages to the surface of the hole transportlayer 130 may be minimized during the formation of the emission layer140 and thus the hole transport layer 130 may maintain excellent holetransporting capability.

In an embodiment, the first hole transporting material may have asubstituted or unsubstituted C4 to C20 alkyl group attached to abackbone structure. When the first hole transporting material has asubstituted or unsubstituted C4 to C20 alkyl group attached to abackbone structure, the first hole transporting material may bedissolved in a non-polar solvent, specifically an aliphatic non-polarsolvent.

In an embodiment, the first hole transporting material may be a compoundrepresented by Chemical Formula 1.

In Chemical Formula 1,

R¹ to R⁸ are each independently hydrogen, a C1 to C3 alkyl group, asubstituted or unsubstituted C4 to C20 alkyl group, a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, a substituted or unsubstituted C3 to C20heteroaryl group, a substituted or unsubstituted C2 to C40 alkylaminegroup, or a substituted or unsubstituted C6 to C40 arylamine group,provided that at least one of R¹ to R⁸ is a substituted or unsubstitutedC4 to C20 alkyl group, and

if any of R³ to R⁸ is a substituted or unsubstituted C3 to C20cycloalkyl group, a substituted or unsubstituted C6 to C20 aryl group,or a substituted or unsubstituted C3 to C20 heteroaryl group, it may beeach independently fused with an adjacent aromatic ring to provide a C8to C15 fused ring,

X¹ and X² are independently selected from N and C(—R^(a)), and X³ and X⁴are independently selected from S, N—R^(b), and C(—R^(c))(—R^(d)),wherein R^(a), R^(b), R^(c), and R^(d) are independently selected fromhydrogen, a substituted or unsubstituted C1 to C20 alkyl group, asubstituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, and a substituted orunsubstituted C3 to C20 heteroaryl group,

L¹ and L² are independently selected from a single bond and asubstituted or unsubstituted methylene group or C2 to C4 alkenylenegroup, and

i, j, k, l, and m are independently 0 or 1. When i, j, k, l, or m is 0,the corresponding moiety represented by it is not present. For example,when j is

and when i is 0,

Since the first hole transporting material includes a compound having arelatively small molecular weight as shown in Chemical Formula 1 and iswell mixed with the ligands on the surface of the quantum dots, theemission layer 140 may exhibit improved surface morphology compared withwhen the quantum dots are included alone and/or the quantum dots and thepolymeric/oligomeric hole transporting material are included.

Specifically, the first hole transporting material may include acompound represented by Chemical Formula 2.

wherein, in Chemical Formula 2,

X³, X⁴, L¹, L², j, k, l, and m are the same as defined in ChemicalFormula 1,

R¹ to R⁸ are each independently hydrogen, a C1 to C3 alkyl group, asubstituted or unsubstituted C4 to C20 alkyl group, a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, a substituted or unsubstituted C3 to C20heteroaryl group, a substituted or unsubstituted C2 to C40 alkylaminegroup, or a substituted or unsubstituted C6 to C40 arylamine group,provided that at least one of R¹ to R⁶ is a substituted or unsubstitutedC4 to C20 alkyl group, and

If any of R³ to R⁶ is one of a substituted or unsubstituted C3 to C20cycloalkyl group, a substituted or unsubstituted C6 to C20 aryl group,or a substituted or unsubstituted C3 to C20 heteroaryl group, it may beeach independently fused with an adjacent aromatic ring to provide a C8to C40 fused ring.

In Chemical Formula 2, at least two of R¹ to R⁶ may be a substituted orunsubstituted C4 to C20 alkyl group. In this case, the solubility of thefirst hole transporting material in the aliphatic non-polar solvent maybe further improved.

For example, the first hole transporting material may include at leastone of compounds represented by Chemical Formula 2-1 to Chemical Formula2-2.

In chemical Formula 2-1 to Chemical Formula 2-2,

R¹¹, R¹², R¹³, R¹⁴, R, R¹⁶, R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are eachindependently hydrogen, a C1 to C3 alkyl group, a substituted orunsubstituted C4 to C20 alkyl group, a substituted or unsubstituted C3to C20 cycloakyl group, a substituted or unsubstituted C6 to C20 arylgroup, or a substituted or unsubstituted C3 to C20 heteroaryl group,provided that at least one of R¹¹ to R¹⁴ and at least one of R²¹ to R²⁶are a substituted or unsubstituted C4 to C20 alkyl group.

R²⁷ and R²⁸ are independently a substituted or unsubstituted C2 to C40alkylamine group, a substituted or unsubstituted C6 to C40 arylaminegroup, or a substituted or unsubstituted carbazolyl group, and

when any of R¹³, R¹⁴, R¹⁵, R¹⁶, R²³, R²⁴, R²⁵, and R²⁶ is one of asubstituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, or a substituted or unsubstitutedC3 to C20 heteroaryl group, it may be each independently fused with anadjacent aromatic ring to provide a C8 to C40 fused ring.

On the other hand, the first hole transporting material may include acompound represented by Chemical Formula 3.

In Chemical Formula 3,

X³, X⁴, L¹, L², j, k, l, and m are independently the same as defined inChemical Formula 1, and

R³³, R³⁴, R³⁵, R³⁶, R³⁷, and R³⁸ are each independently hydrogen, a C1to C3 alkyl group, a substituted or unsubstituted C4 to C20 alkyl group,a substituted or unsubstituted C3 to C20 cycloalkyl group, a substitutedor unsubstituted C6 to C20 aryl group, a substituted or unsubstituted C3to C20 heteroaryl group, a substituted or unsubstituted C2 to C40alkylamine group, or a substituted or unsubstituted C6 to C40 arylaminegroup, provided that at least one of R³³ to R³⁸ is a substituted orunsubstituted C4 to C20 alkyl group, and

when any of R³³, R³⁴, R³⁵, R³⁶, R³⁷, and R³⁸ is one of a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, or a substituted or unsubstituted C3 to C20heteroaryl group, it may be each independently fused with an adjacentaromatic ring to provide a C8 to C40 fused ring.

In Chemical Formula 3, at least two of R³³ to R³⁶ may be anunsubstituted C4 to C10 linear or branched alkyl group. In this case,the solubility of the first hole transporting material in the aliphaticnon-polar solvent may be further improved.

More specifically, the first hole transporting material may include atleast one of compounds represented by Chemical Formula A to ChemicalFormula F.

Hole transporting materials widely used in electroluminescent devicesusing quantum dots have a slower hole mobility than electrontransporting materials. In order to secure stability, such as thelife-span of the quantum dots, an organic ligand is bound to the surfaceof the quantum dots, and the organic ligand attached to the surface ofthe quantum dot generally has a very low hole mobility.

Therefore, at the time of driving the electroluminescent device,electrons and holes may not encounter each other at the center of theemission layer but on the interface with the hole transporting layer orthe hole transporting layer. In this case, the quantum efficiency of thelight emitting layer or the lifetime of the device may be significantlylowered, and therefore, it is necessary to set the electron/hole carrierbalance in the electroluminescent device to an appropriate level.

Accordingly, a method of forming an emission layer by blending quantumdots with conventional hole transporting materials have been considered.However, since the conventional hole transport materials generally usehigher molecular weight materials such as polymers/oligomers, surfacemorphology may not be uniformly formed when they are blended withquantum dots. If the surface morphology of the emission layer is uneven,cracks, pores, and the like may be generated on the surface of theemission layer. Since the cracks and/or pores act as leakage paths ofholes and electrons, leakage currents and turn-on voltages may beincreased as the surface morphology becomes uneven and life-span of adevice may be largely deteriorated.

Further, when spin coating a composition obtained by blending quantumdots with conventional hole transporting materials, the composition maynot be formed into a uniform thin film and may be pushed out, so that aemission layer may not be formed.

On the other hand, the electroluminescent device 10 according to anembodiment includes the emission layer 140 including the plurality ofquantum dots 141 bound to the hydrophobic ligands and the low molecularweight to monomolecular first hole transporting material represented byChemical Formula 1.

As described above, the emission layer 140 composed of the blend of thequantum dots 141 and the low molecular weight to monomolecular firsthole transporting material has a superior surface morphology incomparison with a blend of a conventional blend of hole transportingmaterial and quantum dots. Thus, cracks and/or pores on the surface ofthe emission layer are minimized, thereby reducing the leakage currentand the turn-on voltage, and significantly improving a life-span of thedevice.

In addition, the emission layer 140 composed of the blend of the quantumdots 141 and the low molecular to monomolecular first hole transportingmaterial is easily formed into a uniform thin film using a solutionprocess such as spin coating.

Meanwhile, the electroluminescent device 10 according to an embodimentmay be designed such that the emission layer 140 and the hole transportlayer 130 have relatively different solvent selectivity. Accordingly,damage to the surface of the hole transport layer 130 may be minimizedduring the formation of the emission layer 140, which further reducesthe leakage current and the turn-on voltage.

In addition, since the first hole transporting material is included inthe emission layer 140, hole injection into the emission layer 140 isfacilitated, so that the turn-on of the electroluminescent device 10 maybe accelerated, hole carrier injected into the emission layer 140 may beeasily adjusted, and the electric field voltage applied to the emission140 may be lowered to improve luminous efficiency, maximum luminance andlife-span.

In an embodiment, an amount of the first hole transport material in theemission layer 140 may be controlled considering a desired degree ofhole/electron transportability of the hole transport layer 130 and theelectron transport layer 150, whereby effectively controlling thehole/electron carrier balance in the emission layer 140.

In an embodiment, the first hole transporting material may be forexample included in an amount of greater than or equal to about 2 wt %,greater than or equal to about 5 wt %, greater than or equal to about 10wt %, greater than or equal to about 15 wt %, greater than or equal toabout 20 wt %, greater than or equal to about 25 wt %, or greater thanor equal to about 30 wt % and less than about 50 wt %, for example lessthan or equal to about 49 wt %, less than or equal to about 45 wt %,less than or equal to about 40 wt %, less than or equal to about 30 wt%, or less than or equal to about 25 wt % based on a total weight of theemission layer 140. In an embodiment, the first hole transportingmaterial may be included in an amount of greater than or equal to about2 wt % and less than about 50 wt %, for example greater than or equal toabout 5 wt % and less than about 45 wt %, about 5 wt % to about 40 wt %,about 5 wt % to about 30 wt %, about 5 wt % to about 20 wt % based on atotal weight of the emission layer 140.

When the first hole transporting material is included in the emissionlayer 140 within the above-described range, overall luminous efficiencyand life-span characteristics of the electroluminescent device 10 may begreatly improved.

The surface morphology of the emission layer 140 according to anembodiment may be confirmed by using atomic force microscopy (AFM). Inan embodiment, a root mean square roughness of the emission layer 140measured by the atomic-force microscopy (AFM) may be for example greaterthan or equal to about 0.1 nm (in rms), greater than or equal to about0.2 nm, greater than or equal to about 0.3 nm, greater than or equal toabout 0.4 nm, greater than or equal to about 0.5 nm, greater than orequal to about 0.6 nm, greater than or equal to about 0.7 nm, greaterthan or equal to about 0.8 nm, or greater than or equal to about 0.9 nm,and for example less than or equal to about 2.0 nm, or less than orequal to about 1.5 nm, or for example about 0.5 nm to about 2.0 nm, orabout 0.8 nm to about 1.5 nm.

That is, when the electroluminescent device 10 according to anembodiment forms the emission layer 140 by blending the quantum dots 140and the first hole transporting material, the emission layer 140 mayhave excellent surface morphology as described above.

However, the method of confirming the surface morphology of the lightemitting layer 140 in the electroluminescent device 10 according to anembodiment is not limited to the above-described measuring method. Forexample, the surface morphology may be measured by using othermeasurement methods such as a Zygo interferometer for an emission layerincluding the quantum dots and hole transporting material. In this case,a root mean square roughness range different from the above may beobtained.

However, if the root mean square roughness measured by the measuringmethod according to an embodiment satisfies the above range, theemission layer falls within the scope of the present disclosure.

In an embodiment, a thickness of the emission layer 140 may be selectedconsidering a material used in each of the layers, each electron/holemobility, and a thickness of the hole transport layer 130, or theelectron transport layer 150. The emission layer 140 may have athickness (an average thickness) of about 15 nm to about 100 nm, about20 nm to about 60 nm, about 20 nm to about 50 nm, about 20 nm to about40 nm, or about 25 nm to about 30 nm. The thickness of the emissionlayer 140 according to an embodiment may be adjusted considering arelationship between the materials and/or a thickness of othercomponents. In an embodiment, the thickness of the emission layer 140may be less than or equal to about 60 nm so as to minimize afield-induced quenching, as a measure of reducing a turn-on voltage andan electric field of the emission layer, in consideration of luminousefficiency of the emission layer 140.

Adopting a thickness of the emission layer within the aforementionedrange may avoid formation of voids, cracks, and the like, which may bepresent otherwise in the emission layer 140 to act as leakage paths ofelectrons/holes, making it possible for the carrier balance with otherconstituent elements of the electroluminescent device to be readilymatched.

In addition, within the aforementioned range of a thickness, it becomespossible to readily match the electron/hole carrier balance and toachieve desired current and voltage conditions required for driving andefficient light emission.

Adopting the emission layer of the embodiment may makes it possible forelectrons and holes to be effectively injected into the emission layer140 and to be recombined inside the emission layer 140 rather than on aninterface between the emission layer 140 and the hole transport layer130, whereby the device may be operated without substantial interfacelight emission, and an undesired charge movement into the hole injectionlayer 120 and/or the hole transport layer 150 and a quenching causedthereby may suppressed.

In addition, the electroluminescent device 10 according to an embodimentmay exhibit an improved transporting capability of the emission layer140 and an improved hole transport/injection capability through thefirst hole transport material included in the emission layer 140 in thedevice. As a result, it becomes easier to match the hole/electroncarrier balance in the device.

Accordingly, the electroluminescent device of the embodiment may have amaximum external quantum efficiency (max EQE) of greater than or equalto about 10%, greater than or equal to about 12%, greater than or equalto about 14%, or greater than or equal to about 16%. Theelectroluminescent device of the embodiment may have a maximum luminanceof greater than or equal to about 50,000 cd/m², greater than or equal toabout 55,000 cd/m², greater than or equal to about 60,000 cd/m², greaterthan or equal to about 65,000 cd/m², greater than or equal to about70,000 cd/m², greater than or equal to about 75,000 cd/m², greater thanor equal to about 80,000 cd/m², greater than or equal to about 85,000cd/m², greater than or equal to about 90,000 cd/m², greater than orequal to about 95,000 cd/m², greater than or equal to about 100,000cd/m², greater than or equal to about 105,000 cd/m², greater than orequal to about 110,000 cd/m², greater than or equal to about 115,000cd/m², or greater than or equal to about 120,000 cd/m².

The electroluminescent device of the embodiment may have a T95 (or T90or T80) of greater than or equal to about 100 hours, greater than orequal to about 150 hours, greater than or equal to about 200 hours,greater than or equal to about 250 hours, greater than or equal to about300 hours, greater than or equal to about 400 hours, greater than orequal to about 450 hours, greater than or equal to about 500 hours,greater than or equal to about 550 hours, greater than or equal to about600 hours, greater than or equal to about 650 hours, at the operation of1500 nit. When an AC voltage is applied, the electroluminescent deviceof the embodiment may have a maximum capacitance of greater than orequal to about 0.8 nF, greater than or equal to about 1 nF, and/or lessthan or equal to about 50 nF (e.g., at 100 Hz).

The electroluminescent device of an embodiment may further include anelectron transport layer 150 disposed between the emission layer 140 andthe second electrode 160, transporting electrons into the emission layer140.

In an embodiment, an average thickness of the electron transport layer150 may be variously changed considering charge carrier balance of thehole injection layer 120, the hole transport layer 130, and/or theemission layer 140 in the device, but may be for example greater than orequal to about 20 nm, greater than or equal to about 30 nm, greater thanor equal to about 40 nm, or greater than or equal to about 50 nm, andfor example less than or equal to about 100 nm, less than or equal toabout 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, or less than or equal to about 60 nm, for example about 20nm to about 100 nm, about 20 nm to about 90 nm, about 30 nm to about 80nm, about 40 mm to about 80 nm, or about 60 nm to about 80 nm.

The emission layer of the embodiment may not include an electrontransporting material that will be described below.

In an embodiment, the electron transport layer 150 may include a nonlight-emissive electron transporting materials, and it does not emitlight on the application of an electric field and the electrons may notbe quenched internally.

The electron transport layer 150 may include inorganic materialnanoparticles or may be an organic layer formed by deposition. Forexample, the electron transport layer 150 may include inorganicnanoparticles, quinolone compounds, triazine compounds, quinolinecompounds, triazole compounds, naphthalene compounds, or a combinationthereof.

In an embodiment, the electron transport layer 150 may include inorganicnanoparticles 151. The inorganic nanoparticles 151 impart an electrontransporting property to the electron transport layer 150 and do notexhibit luminescent properties.

In an embodiment, the electron transport layer 150 may include at leasttwo inorganic nanoparticles 151. In an embodiment, the electrontransport layer 150 may include a cluster layer composed of at least twoinorganic nanoparticles 151.

The inorganic nanoparticles may include a compound including a zincmetal oxide represented by Zn_(1-x)M_(x)O (wherein M is Mg, Ca, Zr, W,Li, Ti, Y, Al, or a combination thereof, and 0≤x≤0.5). in an embodiment,M may be Mg and the x may be greater than or equal to about 0.01 andless than or equal to about 0.3, for example, less than or equal toabout 0.25, less than or equal to about 0.2, or less than or equal toabout 0.15.

The inorganic nanoparticles may include a zinc oxide, a zinc magnesiumoxide, or a combination thereof. An average size of the nanoparticle maybe greater than or equal to about 1 nm, greater than or equal to about1.5 nm, greater than or equal to about 2 nm, greater than or equal toabout 2.5 nm, or greater than or equal to about 3 nm and less than orequal to about 10 nm, less than or equal to about 9 nm, less than orequal to about 8 nm, less than or equal to about 7 nm, less than orequal to about 6 nm, or less than or equal to about 5 nm. Thenanoparticle may not have a rod shape.

The nanoparticle may not have a nanowire shape. An electron injectionlayer facilitating injection of electrons and/or a hole blocking layerblocking movement of holes may be further disposed between the electrontransport layer 150 and the second electrode 160.

The electron injection layer and the hole blocking layer may have eachdesirably selected thickness. For example, each thickness may be in arange of greater than or equal to about 1 nm and less than or equal toabout 500 nm but is not limited thereto. The electron injection layermay be an organic layer formed through deposition.

The electron injection layer may include for example at least one of1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl] borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, ET204(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),8-hydroxyquinolinato lithium (Liq), n-type metal oxide (e.g., ZnO, HfO₂,etc.), ET204(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),Bphen, ABH113, NET218, NET338, NDN77, NDN87, or a combination thereof,but is not limited thereto.

The hole blocking layer (HBL) may include for example at least one of1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl] borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, or a combination thereof, but is notlimited thereto.

As described above, the electroluminescent device 10 according to anembodiment may improve hole mobility of the emission layer 140 byincluding the quantum dots and the first hole transporting material inthe emission layer 140. In addition, the first hole transportingmaterial in the emission layer is a monomolecular to low molecularweight material having a substituted or unsubstituted C4 to C20 alkylgroup attached to a backbone structure, and thus a solubility in anon-polar solvent is improved and the formed emission layer may haveimproved surface morphology when the light emitting layer comprising theplurality of quantum dots and the first hole transporting material areformed using a solution process.

In addition, according to an embodiment, the emission layer 140 and thehole transport layer 130 may be controlled to have solvent selectivityas described above. Accordingly, because the surface of the holetransport layer 130 is not damaged, the emission layer 140 and the holetransport layer 130 may maintain excellent hole transporting capability.

In addition, since the first hole transporting material included in theemission layer 140 has a substituted or unsubstituted C4 to C20 alkylgroup attached to a backbone structure, it may be dissolved in thealiphatic non-polar solvent together with quantum dots on whichhydrophobic ligands are attached. Accordingly, when the emission layer140 is formed using the solution, surface morphology of the formedemission layer 140 is improved and hole transporting capability isimproved due to the first hole transporting material. As a result, sincehole transporting capability of the emission layer 140 itself isimproved and conductivity of the emission layer 140 is also improved,quenching caused by an electric field of the emission layer 140 may bedecreased and thus luminous efficiency, luminance, and life-span of theelectroluminescent device 10 may be largely improved.

Hereinafter, a display device including the electroluminescent device 10is described.

A display device according to an embodiment includes a substrate, adriving circuit formed on the substrate, and a first electroluminescentdevice, a second electroluminescent device, and a thirdelectroluminescent device spaced apart from each other in apredetermined interval and disposed on the driving circuit.

The first to third electroluminescent devices have the same structure asthe electroluminescent device 10 and but the wavelengths of the lightsemitted from each quantum dots may be different from each other.

In an embodiment, the first electroluminescent device is a red deviceemitting red light, the second electroluminescent device is a greendevice emitting green light, and the third electroluminescent device isa blue device emitting blue light. In other words, the first to thirdelectroluminescent devices may be pixels expressing red, green, andblue, respectively, in the display device.

However, an embodiment is not necessarily limited thereto, but the firstto third electroluminescent devices may respectively express magenta,yellow, cyan, or may express other colors.

One of the first to third electroluminescent devices may be theelectroluminescent device 10. In this case, the third electroluminescentdevice displaying at least blue may be desirably the electroluminescentdevice 10.

In the display device according to an embodiment, a hole injectionlayer, a hole transport layer, an electron transport layer, an electroninjection layer, and a hole blocking layer except an emission layer ofeach pixel may be integrated to form a common layer. However, anembodiment is not limited thereto. A hole injection layer, a holetransport layer, an electron transport layer, an electron injectionlayer, and a hole blocking layer may be independently formed in eachpixel of the display device, or at least one of a hole injection layer,a hole transport layer, an electron transport layer, an electroninjection layer, and a hole blocking layer may form a common layer andremaining layers may form a separate independent layer.

The substrate may be a transparent insulating substrate or may be madeof a ductile material. The substrate may include glass or a polymermaterial in a film having a glass transition temperature (Tg) of greaterthan about 150° C. For example, it includes a COC (cycloolefincopolymer) or COP (cycloolefin polymer) material. All the first to thirdelectroluminescent devices are formed on the substrate. That is, asubstrate of the display device according to an embodiment provides acommon layer.

The driving circuit is disposed on the substrate and is independentlyconnected to each of the first to third electroluminescent devices. Thedriving circuit may include at least one kind of line including a scanline, a data line, a driving power source line, a common power sourceline, and the like, at least two of thin film transistors (TFT)connected to the wire and corresponding to one organic light emittingdiode, and at least one capacitor, or the like. The driving circuit mayhave a variety of the known structures.

As described above, a display device according to an embodiment mayexhibit improved device efficiency and thus excellent photoluminescencecharacteristics.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent disclosure is not limited thereto.

EXAMPLES Preparation Example 1

5×10⁻⁵ millimoles (mmol) of red quantum dots (InP, average particlediameter: 9 nm) on which oleic acid is attached as a hydrophobic ligandand 2.66 mmol of a compound represented by Chemical Formula D (DOFL-TPD)are put in 10 mL of octane and stirred for 5 minutes, preparing acomposition for an emission layer.

The compound represented by Chemical Formula D is included in an amountof 5 wt % based on a total weight of the quantum dots and the compoundrepresented by Chemical Formula D.

Preparation Example 2

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except that the amount of the compoundrepresented by Chemical Formula D is adjusted into 10 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula D.

Preparation Example 3

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except that the amount of the compoundrepresented by Chemical Formula D is adjusted into 15 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula D.

Preparation Example 4

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except that the amount of the compoundrepresented by Chemical Formula D is adjusted into 20 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula D.

Preparation Example 5

3.33×10⁻⁵ mmol of blue quantum dots (ZnSeTe, average particle diameter:13 nm) on which oleic acid is attached as a hydrophobic ligand and 2.66mmol of a compound represented by Chemical Formula D (DOFL-TPD) are putin 10 mL of octane and stirred for 5 minutes, preparing a compositionfor an emission layer.

The compound represented by Chemical Formula D is included in an amountof 15 wt % based on a total weight of the quantum dots and the compoundrepresented by Chemical Formula D.

Preparation Example 6

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except using 3.64 mmol of the compoundrepresented by Chemical Formula B (DOFL-NPB) instead of the compoundrepresented by Chemical Formula D (DOFL-TPD) and adjusting the amount ofthe compound represented by Chemical Formula B into 5 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Preparation Example 7

A composition for an emission layer is prepared according to the samemethod as Preparation Example 6 except that the amount of the compoundrepresented by Chemical Formula B is adjusted into 10 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Preparation Example 8

A composition for an emission layer is prepared according to the samemethod as Preparation Example 6 except that the amount of the compoundrepresented by Chemical Formula B is adjusted into 15 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Preparation Example 9

A composition for an emission layer is prepared according to the samemethod as Preparation Example 6 except that the amount of the compoundrepresented by Chemical Formula B is adjusted into 20 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Preparation Example 10

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except that the amount of the compoundrepresented by Chemical Formula D is adjusted into 30 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula D.

Preparation Example 11

A composition for an emission layer is prepared according to the samemethod as Preparation Example 6 except that the amount of the compoundrepresented by Chemical Formula B is adjusted into 40 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Preparation Example 12

Red light emitting core-shell quantum dots (InP/ZnSeS, average particlediameter: 11 nm) on a surface of which oleic acid is attached as ahydrophobic ligand and a compound represented by Chemical Formula D(DOFL-TPD) are put in 10 mL of octane and stirred for 5 minutes,preparing a composition for an emission layer (concentration: 5×10⁻⁵mmol).

The compound represented by Chemical Formula D is included in an amountof 15 wt % based on a total weight of the quantum dots and the compoundrepresented by Chemical Formula D.

Comparative Preparation Example 1

5×10⁻⁵ mmol of red quantum dots (InP, an average particle diameter: 9nm) on which oleic acid is attached as a hydrophobic ligand are put in10 mL of octane and stirred for 5 minutes, preparing a composition foran emission layer.

Comparative Preparation Example 2

3.33×10⁻⁵ mmol of blue quantum dots (ZnTeSe, an average particlediameter: 13 nm) on which oleic acid is attached as a hydrophobic ligandare added to 10 mL of octane and stirred for 5 minutes, preparing acomposition for an emission layer.

Comparative Preparation Example 3

A composition for an emission layer is prepared according to the samemethod as Preparation Example 6 except that the amount of the compoundrepresented by Chemical Formula B is adjusted into 50 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula B.

Comparative Preparation Example 4

A composition for an emission layer is prepared according to the samemethod as Preparation Example 1 except that the amount of the compoundrepresented by Chemical Formula D is adjusted into 50 wt % based on atotal weight of the quantum dots and the compound represented byChemical Formula D.

Comparative Preparation Example 5

A composition for an emission layer is prepared according to the samemanner as Preparation Example 12 except that the amount of the compoundrepresented by Chemical Formula D is not used.

Comparative Preparation Example 6

A composition for an emission layer is prepared according to the samemanner as Preparation Example 12 except that instead of the compound(DOFL-TPD) represented by Chemical Formula D, a compound having thefollowing structure is used:

Evaluation 1: Surface Morphology of Emission Layer

The compositions for an emission layer according to Preparation Examples3 and 5 and Comparative Preparation Examples 1 and 2 are respectivelyspin-coated on a glass substrate, thermally treated at 80° C. for 30minutes to form each emission layer thin film having an averagethickness of about 25 nm.

Subsequently, surface morphology of each emission layer thin film ismeasured through atomic-force microscopy (AFM), and the results areshown in FIGS. 2 to 5.

FIGS. 2 to 5 are atomic-force microscopic (AFM) images showing surfacemorphology of the emission layer thin films (Comparative PreparationExample 1 (FIG. 2), Preparation Example 3 (FIG. 3), ComparativePreparation Example 2 (FIG. 4), and Preparation Example 5 (FIG. 5)).

In FIGS. 2 to 5, each image has each width scale corresponding to 5 μm,and herein, as an image shows a brighter color, it is positionedrelatively upper, but as it shows a darker color, it is positionedrelatively lower.

Referring to FIGS. 2 to 5, the emission layer thin films in which a holetransporting material is blended based on the same kind of quantum dotsaccording to Preparation Examples 3 and 5 show lower root mean squareroughness (RMS) compared with those according to Comparative PreparationExamples 1 and 2 including no hole transporting material.

The results of FIGS. 2 to 5 show that an emission layer formed byblending a hole transporting material with quantum dots according toembodiment shows excellent surface morphology compared with an emissionlayer including only the quantum dots.

Evaluation 2: Hole Transporting Capability Evaluation of HOD

The compositions for an emission layer according to Preparation Examples2 to 3 and 6 to 8 and Comparative Preparation Example 1 are respectivelyused to manufacture HOD (Hole Only Device). HOD may be manufactured byrespectively spin-coating a solution for a hole injection layer preparedby dissolving PEDOT in water and a solution for a hole transport layerprepared by dissolving TFB in toluene on a patterned ITO electrode tosequentially form a hole injection layer and a hole transport layer,coating and drying a composition for an emission layer thereon to forman emission layer, depositing a material blocking holes passing theemission layer on the emission layer, and finally forming an Alelectrode. The manufactured HOD may effectively sense only holetransporting capability.

Subsequently, after applying a voltage of 5 V and 8 V to HOD, currentdensity measured in each emission layer is shown in Table 1.

TABLE 1 Comparative Applied Preparation Preparation PreparationPreparation Preparation Preparation Voltage Example 2 Example 3 Example6 Example 7 Example 8 Example 1 5 V 0.08 mA/cm² 0.67 mA/cm² 0.06 mA/cm²0.47 mA/cm²  0.77 mA/cm²  0.01 mA/cm² 8 V 2.32 mA/cm² 10.1 mA/cm² 3.50mA/cm² 9.63 mA/cm² 15.96 mA/cm² 0.025 mA/cm²

Referring to Table 1, HOD's manufactured by using the compositions foran emission layer according to the Preparation Examples show improvedcurrent density compared with HOD manufactured by using the compositionsfor an emission layer according to Comparative Preparation Example 1.

On the other hand, when the same kind of hole transporting material(comparison of Preparation Examples 2 and 3 and comparison ofPreparation Examples 6 to 8) is used, as an amount of the holetransporting material is increased, current density also increases.

However, when different kinds of hole transporting materials (comparisonof Preparation Examples 2 to 3 and Preparation Examples 6 to 8) is used,current density also becomes different.

Referring to the result of Table 1, HOD's according to PreparationExamples show much improved hole transporting capability through theemission layer formed by blending quantum dots and a hole transportingmaterial.

Evaluation 3: Surface Characteristics of Emission Layer Depending onContent of First Hole Transport Material

The compositions for an emission layer according to Preparation Examples10 to 11 and Comparative Preparation Examples 3 to 4 are respectivelycoated and dried on a glass substrate to form emission layer thin films,the surface of each emission layer thin films is measured by using ascanning electron microscope, and the results are shown in FIGS. 6 to 9.

FIGS. 6 to 9 are scanning electron microscopic images of the emissionlayer thin films (Comparative Preparation Example 3 (FIG. 6),Comparative Preparation Example 4 (FIG. 7), Preparation Example 10 (FIG.8), and Preparation Example 11 (FIG. 9)).

Referring to FIGS. 6 to 9, the emission layers according to PreparationExamples 10 to 11 show a relatively uniform surface distribution, butwhen a first hole transporting material is excessively included, a phaseseparation may occur as shown in Comparative Preparation Examples 3 to4.

Accordingly, referring to the result of Evaluation 3, theelectroluminescent devices according to Examples include a first holetransporting material in an emission layer within the above range andthus may prevent a phase separation on the surface of the emission layerand secure excellent surface roughness.

Evaluation 3-1: Time Resolved Photoluminescence Analysis of the EmissionLayer

Using the compositions of Preparation Example 12 and ComparativePreparation Example 5, Emission Film 1 (QD:TPD film) and Emission Film 2(QD only film) are formed, respectively. DOFL-TPD (hereinafter, also maybe abbreviated as “TPD” e.g., Table 2) is dissolved in octane to form asolution, which is then used to form a TPD film.

Using fluorescence lifetime spectrometer (“picoquant fluotime 300” byPicoQuant Co. Ltd.), a TRPL analysis is made for each of Emission Film1, Emission Film 2, and TPD Film and the results are summarized in Table2, FIG. 14 and FIG. 15.

TABLE 2 QD + TPD QD + TPD 15% QD 15% TPD film λ_(probe) TPD onlyλ_(probe) QD t1 0.8 (100%) 0.3 (100%) 9 (0%)  9 (24%) t2 — — 27 (95%) 27(72%) t3 — — 110 (5%)  110 (4%)  t_(avg) 0.8 0.3 31 26

The results of Table 2 and FIGS. 14 and 15 confirm that the decay timeof the first hole transporting material is significantly reduced by amixing with the quantum dots and that for the excitons related with thequantum dot emission, a component of a very fast decay (fast initialdecay) appears. The results indicate that the decay path of the exitonsin Emission Film 1 becomes different (changes) from that in EmissionFilm 2.

Evaluation 3-2: Photoluminescence Excitation Analysis of the EmissionFilm

Using Fluorescence Spectrophotometer (Hitachi F7000), photoluminescentemission behavior of Emission Film 1 (QD:TPD film) and Emission Film 2(QD only film) is observed with varying a wavelength of excitation lightfrom 300 nm to 600 nm and the results are summarized in FIG. 16.

From the results of FIG. 16, the photoluminescent emission behavior inEmission Film 2 is clearly different from that in Emission Film 1. InEmission Film 1, a contribution rate of the quantum dots themselves forthe red light emission is about 40% while a contribution rate of thequantum dots and the TPD is about 60%.

In Emission Film 1, when the wavelength of the excitation light is thephotoluminescent wavelength of the TPD (i.e., 422 nm), the intensity ofthe emitted light is less than about 0.4 (e.g., less than or equal toabout 0.2) of the maximum intensity the emitted light (i.e., 1). Whenthe wavelength of the excitation light is 450 nm, the intensity of theemitted light is less than or equal to about 0.1 of the maximumintensity the emitted light.

Evaluation 3-3: Photoluminescence Analysis of the Emission FilmCompositions are prepared in the same manner as Preparation Example 12except that the amount of the DOFL-TPD is changed into 0 wt %, 5 wt %,10 wt %, 15 wt %, and 20 wt %, respectively and using each of theprepared composition, a emission film is prepared.

Using Fluorescence Spectrophotometer (Hitachi F7000), a photoluminescentcharacteristic is analyzed for each of the prepared emission film withexcitation light having a wavelength of 380 nm or 530 nm, and theresults (a relative intensity of emission calculated from a spectrumarea) are summarized in Table 3.

TABLE 3 Relative intensity Relative intensity of emission (%) ofemission (%) at excitation at excitation of 380 nm of 530 nm QD only100% 100%  TPD 5% 113% 85% TPD 10% 142% 83% TPD 15% 129% 83% TPD 20%141% 77%

With an excitation light of 380 nm, an increase of the amount of TPDbrings forth an increase of the photoluminescence of the QDs. With anexcitation light of 530 nm, only the QDs are excited and thus anincrease of the amount of TPD brings forth a decrease of the PL, this isconsistent with a decrease of the number of the QDs.

Example 1

A glass substrate deposited with ITO as a first electrode (an anode) issurface-treated with UV-ozone for 15 minutes, a hole injection layerhaving an average thickness of 30 nm is formed thereon by spin-coating aPEDOT:PSS solution (H.C. Starks) and thermally treating it at 150° C.for 10 minutes under an air atmosphere and then, at 150° C. for 10minutes under a N₂ atmosphere.

Subsequently, a hole transport layer having an average thickness of 25nm is formed on the hole injection layer by spin-coating a solution fora hole transport layer prepared by dissolvingpoly[(9,9-dioctylfluoren-2,7-diyl)-co-(4,4′-(N-4-butylphenyl)diphenylamine)](TFB) (Sumitomo) in toluene and thermally treating it at 150° C. for 30minutes.

Subsequently, the composition for an emission layer according toPreparation Example 1 is spin-coated on the hole transport layer andheated at 80° C. to form a red emission layer (red light emitting layer)having an average thickness of 25 nm.

On the red emission layer, an electron transport layer having an averagethickness of 80 nm is formed by spin-coating a solution for an electrontransport layer in which ZnO is dispersed in ethanol and then, thermallytreating it at 80° C. for 30 minutes.

Subsequently, a second electrode is formed by vacuum-depositing aluminum(Al) to an average thickness of 90 nm on the electron transport layer tomanufacture an electroluminescent device according to Example 1.

Example 2

An electroluminescent device according to Example 2 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 2 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 3

An electroluminescent device according to Example 3 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 3 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 4

An electroluminescent device according to Example 4 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 4 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 5

An electroluminescent device according to Example 5 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 2 is used insteadof the composition for an emission layer according to PreparationExample 1 and the electron transport layer is formed with an averagethickness of 60 nm.

Example 6

An electroluminescent device according to Example 6 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 3 is used insteadof the composition for an emission layer according to PreparationExample 1 and the electron transport layer is formed with an averagethickness of 60 nm.

Example 7

An electroluminescent device according to Example 7 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 3 is used insteadof the composition for an emission layer according to PreparationExample 1, the emission layer is formed with an average thickness of 30nm, and the electron transport layer is formed with an average thicknessof 60 nm.

Example 8

An electroluminescent device according to Example 8 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 3 is used insteadof the composition for an emission layer according to PreparationExample 1 and the emission layer is formed with an average thickness of30 nm.

Example 9

An electroluminescent device according to Example 9 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 6 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 10

An electroluminescent device according to Example 10 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 7 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 11

An electroluminescent device according to Example 11 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 8 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 12

An electroluminescent device according to Example 12 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 9 is used insteadof the composition for an emission layer according to PreparationExample 1.

Example 13

An electroluminescent device according to Example 13 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 7 is used insteadof the composition for an emission layer according to PreparationExample 1 and the electron transport layer is formed with an averagethickness of 60 nm.

Example 14

An electroluminescent device according to Example 14 is manufacturedaccording to the same method as Example 1, except that the compositionfor an emission layer according to Preparation Example 8 is used insteadof the composition for an emission layer according to PreparationExample 1 and the electron transport layer is formed with an averagethickness of 60 nm.

Example 15

The composition for an emission layer according to Preparation Example15 is spin-coated on the hole transport layer and heated at 80° C. toform a red light emission layer having an average thickness of 20 nm.

A solution for an electron transport layer having ZnMgO nanoparticlesdispersed in ethanol is spin-coated on the red light emission layer, andthen is thermally treated at 80° C. for 30 minutes to form an electrontransport layer having an average thickness of 40 nm.

Then, a second electrode is formed by vacuum-depositing aluminum (Al) toan average thickness of 100 nm on the electron transport layer to obtainan electroluminescent device.

Comparative Example 1

An electroluminescent device according to Comparative Example 1 ismanufactured according to the same method as Example 1, except that thecomposition for an emission layer according to Comparative PreparationExample 1 is used instead of the composition for an emission layeraccording to Preparation Example 1.

Comparative Example 2

An electroluminescent device according to Comparative Example 2 ismanufactured according to the same method as Example 1, except that thecomposition for an emission layer according to Comparative PreparationExample 2 is used instead of the composition for an emission layeraccording to Preparation Example 1 and the electron transport layer isformed with an average thickness of 60 nm.

Comparative Example 3

An electroluminescent device is manufactured in the same manner asExample 12, except that the composition of Comparative PreparationExample 5 is used instead of the composition of Preparation Example 12.

Comparative Example 4

An electroluminescent device is manufactured in the same manner asExample 12, except that the composition of Comparative PreparationExample 6 is used instead of the composition of Preparation Example 12.

A serious phase segregation occurs during the formation of the emissionlayer and the manufactured device does not show a luminescentcharacteristic.

Evaluation 4: Voltage-Current Density Relationship Depending on Contentof Hole Transporting Material in Emission Layer

Voltage-current density relationship of the electroluminescent devicesaccording to Examples 1 to 4 and Comparative Example 1 and of theelectroluminescent devices according to Examples 9 to 12 and ComparativeExample 2 are measured and respectively shown in FIGS. 10 and 11.

FIG. 10 is a graph showing voltage-current density (log scale)characteristics of the electroluminescent devices according to Examples1 to 4 and Comparative Example 1 and FIG. 11 is a graph showingvoltage-current density (log scale) characteristics of theelectroluminescent devices according to Examples 9 to 12 and ComparativeExample 2.

Referring to FIGS. 10 and 11, a turn-on voltage of a device is graduallylower, as an amount of a hole transporting material is increased in anemission layer under the predetermined conditions (i.e., same kind ofemission layer and same thickness of an electron transport layer of FIG.10, and Examples 9 to 12 of FIG. 11).

Referring to the results of FIGS. 10 and 11, the electroluminescentdevices according to Examples show improved hole transporting capabilitycompared with those according to Comparative Examples, and as an amountof the hole transporting material is increased in the emission layer,the hole transporting capability is gradually improved.

Evaluation 5: Relationship of Average Thickness of Electron TransportLayer and Device Characteristics

Device characteristics of the electroluminescent devices according toExamples 2, 6, 11, 13 and Comparative Examples 1 and 2 are measured andshown in Table 4.

TABLE 4 EQE EQE EQE Max Driving Luminance @ @ @ Current Voltage @ 5Lamda Max Max. 100 500 1000 efficiency (V) @ 5 mA/cm² max. FWHMLuminance EQE nt nt nt (Cd/A) mA/cm² (Cd/m²) (nm) (nm) CIE x CIE y(Cd/m²) Example 2 3.6 2.3 3.3 3.6 4.5 2.6 175 627 37 0.6830 0.3150 17240Example 6 4.1 2.8 3.8 4.0 5.3 2.8 220 626 37 0.6820 0.3161 16230 Example11 3.5 2.6 3.3 3.5 4.3 2.7 178 627 37 0.6850 0.3140 14040 Example 13 5.23.7 4.9 5.1 6.6 2.8 301 626 37 0.6820 0.3160 10780 Comparative 3.8 2.23.7 3.6 4.7 3.0 182 627 37 0.6840 0.3140 6850 Example 1 Comparative 4.23.0 3.9 4.1 5.3 2.8 228 627 37 0.6830 0.3160 7170 Example 2 nt = nit

Referring to Table 2, based on the average thickness of the same kind ofelectron transport layers (comparison of Examples 2 and 11 andComparative Example 1 and comparison of Examples 6 and 13 andComparative Example 2), specific device characteristics may be a littledifferent depending on a kind of hole transporting material included inan emission layer, but Examples show remarkably excellent maximumluminance (Max luminance) compared with at least Comparative Examples.On the other hand, based on the same hole transporting material(comparison of Examples 2 and 6 and comparison of Examples 11 and 13),device characteristics such as external quantum efficiency (EQE),maximum luminance (Max luminance), or the like may be differentdepending on an average thickness of the electron transport layer (ETL).

On the other hand, life-span characteristics of the electroluminescentdevices according to Examples 2, 5, 11, and 14 and Comparative Examples1 and 2 are respectively evaluated and the results are shown in Table 5and FIG. 8.

FIG. 12 is a graph showing life-span (i.e., lifetime) characteristics ofthe electroluminescent devices (at 500 nit) according to Examples 2, 5,11, and 14 and Comparative Examples 1 and 2.

TABLE 5 T95 (h) T50 (h) Example 2 0.09 greater than 1,000 Example 5 0.09greater than 1,000 Example 11 154.91 about 700 Example 14 34.35 greaterthan 1,000 Comparative Example 1 0.03 4.8 Comparative Example 2 0.03 0.9

T95 indicates the time (h) clasped until the luminance is 95% of aninitial luminance (100%), and T50 indicates the time (h) clasped untilthe luminance is 50% of an initial luminance (100%).

Referring to FIG. 12 and Table 3, based on the same thickness of theelectron transport layers (comparison of Examples 2 and 11 andComparative Example 1 and comparison of Examples 5 and 14 andComparative Example 2), the electroluminescent devices according toExamples show excellent T50 and T95 compared with those of ComparativeExamples. On the other hand, based on the same hole transportingmaterial (comparison of Examples 2 and 5 with Comparative Example 1,comparison of Examples 11 and 14 with Comparative Example 2), life-spancharacteristics may also be different depending on an average thicknessof an electron transport layer (particularly, referring to Examples 11and 14).

Accordingly, referring to the result of Evaluation 5, the electrontransport layer may have a different optimal average thickness dependingon a kind of a hole transporting material included in an emission layer.

Evaluation 6: Relationship Between Average Thickness of ElectronTransport Layer and Emission Layer and Device Characteristics

Device characteristics of the electroluminescent devices according toExamples 3, 6, 7, and 8 and Comparative Examples 1 and 2 are measured,and the results are shown in Table 6.

TABLE 6 EQE EQE EQE Max Driving Luminance @ @ @ Current Voltage @ 5Lamda Max Max. 100 500 1000 efficiency (V) @ 5 mA max. FWHM LuminanceEQE nt nt nt (Cd/A) mA (Cd/m²) (nm) (nm) CIE x CIE y (Cd/m²) Example 33.6 2.3 3.3 3.6 4.5 2.6 175 627 37 0.6830 0.3150 17240 Example 6 4.1 2.83.8 4.0 5.3 2.8 220 626 37 0.6820 0.3161 16230 Example 7 2.8 2.2 2.6 2.83.6 2.7 149 626 37 0.6820 0.3160 15510 Example 8 2.8 2.4 2.7 2.8 3.4 2.7156 628 38 0.6850 0.3140 19560 Comparative 3.8 2.2 3.7 3.6 4.7 3.0 182627 37 0.6840 0.3140 6850 Example 1 Comparative 4.2 3.0 3.9 4.1 5.3 2.8228 627 37 0.6830 0.3160 7170 Example 2

Referring to Table 4, based on the same average thickness of theelectron transport layers (comparison of Examples 3 and 8 withComparative Example 1 and comparison of Examples 6 and 7 withComparative Example 2), maximum luminance depending on the averagethickness of the emission layer may be opposite each other. In otherwords, as for Examples 3 and 8, as the average thickness of the emissionlayer is increased, the maximum luminance increases, but as for Examples6 and 7, the maximum luminance rather decreases with increased emissionlayer thickness. On the other hand, life-span characteristics of theelectroluminescent devices according to Examples 3, 6, 7, and 8 andComparative Examples 1 and 2 are respectively evaluated, and the resultsare shown in Table 7 and FIG. 13.

FIG. 13 is a graph showing life-span characteristics of theelectroluminescent devices according to Comparative Examples 1 to 2 andExamples 3, 6, 7, and 8.

TABLE 7 T95 (h) T50 (h) Example 3 0.09 greater than 1,000 Example 6 0.09greater than 1,000 Example 7 0.07 greater than 1,000 Example 8 1.02greater than 1,000 Comparative Example 1 0.03 4.8 Comparative Example 20.03 0.9

Referring to FIG. 13 and Table 5, based on the same thickness of theelectron transport layers (comparison of Examples 3 and 8 withComparative Example 1 and comparison of Examples 6 and 7 withComparative Example 2), the electroluminescent device according toExamples show excellent T50 and T95 compared with those according toComparative Examples. On the other hand, based on the same holetransporting material and the same average thickness of the electrontransport layers (comparison of Examples 6 and 7 and comparison ofExamples 3 and 8) and particularly, referring to T95 of Examples 7 and8, life-span characteristics may be different, as a thickness of theemission layer is changed. However, life-span characteristics ofExamples 3, 6, 7, and 8 show excellent T50 of greater than 1,000 hours.

Accordingly, referring to the result of Evaluation 6, an averagethickness of the emission layer has an influence on devicecharacteristics, and accordingly, the device characteristics may beadjusted by controlling an average thickness of the electron transportlayer and an average thickness of the emission layer.

Evaluation 7: Electroluminescent Properties and Lifetime of theManufactured Device

Device characteristics and lifetime of the electroluminescent devices ofExample 15 and Comparative Example 3 are measured, and the results aresummarized in Table 8.

T80(h): time (in hours) taken to reach relative luminance 80% relativeto initial luminance when the device is operated at 1500 nit ismeasured.

T95(h): time (in hours) taken to reach relative luminance 95% relativeto initial luminance when the device is operated at 1500 nit is measured

TABLE 8 EQE Lum Lamda Max Max Max T95 (h) T80 (h) (%) (cd/m²) (nm) @1500nit @1500 nit Comp. 15.5 112920 631 83.9 465.8 Example 3 Example15 16.5127760 631 484.4 1619.8

The results of Table 8 confirm that the electroluminescent device of theExample shows an increased lifetime together with improvedelectroluminescent properties.

Evaluation 8: The Device Property During the Application of an ACVoltage

A maximum capacitance over the changes of the voltage is measured whilean AC voltage is applied to the electroluminescent device of Example 15and Comparative Example 3 and the results are summarized in Table 9.

TABLE 9 Maximum Capacitance (nF) Example15 11 Comp. 7 Example 3

The results of the tests confirm that for the device of Example 15, adriving voltage may slightly decrease and a charge accumulated at aninterface of HTL/QD may be changed and a luminance at a low voltage mayincrease. The results also indicate an improved mobility in the mixedemission layer.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. An electroluminescent device, comprising a firstelectrode and a second electrode facing each other; an emission layerdisposed between the first electrode and the second electrode andcomprising: a plurality of quantum dots, and a first hole transportingmaterial, wherein the quantum dots are configured to emit a first lightupon excitation by light, and the first hole transporting material isconfigured to emit a second light upon excitation by light, wherein thefirst light has a first peak wavelength and the second light has asecond peak wavelength, and wherein in a time resolved photoluminescenceanalysis of the emission layer, t1 for the first light is less than orequal to about 15 nanoseconds.
 2. The electroluminescent device of claim1, wherein the t1 for the first light is less than or equal to about 10nanoseconds.
 3. The electroluminescent device of claim 1, wherein in atime resolved photoluminescence analysis of the emission layer, a decayfraction at the t1 for the first light is greater than or equal to about5%.
 4. The electroluminescent device of claim 1, wherein in a timeresolved photoluminescence analysis of the emission layer, an averagedecay time (t_(avg)) for the first light is less than or equal to about30 nanoseconds.
 5. The electroluminescent device of claim 1, wherein ina time resolved photoluminescence analysis of the emission layer, anaverage decay time (t_(avg)) for the second light is less than or equalto about 0.7 nanoseconds.
 6. The electroluminescent device of claim 1,wherein in an analysis for an excitation wavelength dependentphotoluminescence emission behavior, the emission layer shows aluminance intensity that is less than or equal to about 0.4 of a maximumintensity thereof, when an excitation light has a wavelength of thesecond peak wavelength.
 7. The electroluminescent device of claim 1,wherein the second peak wavelength is less than the first peakwavelength.
 8. The electroluminescent device of claim 1, wherein on anapplication of a voltage, the emission layer emits light of the samecolor as the first light.
 9. The electroluminescent device of claim 1,wherein the emission layer is configured to emit light without aformation of a triplet emitter.
 10. The electroluminescent device ofclaim 1, wherein the emission layer does not include an iridiumcontaining organic metal compound, a platinum containing organic metalcompound, or a combination thereof.
 11. The electroluminescent device ofclaim 1, wherein the first hole transporting material comprises asubstituted or unsubstituted C4 to C20 alkyl group attached to abackbone structure.
 12. The electroluminescent device of claim 1,wherein the first hole transporting material comprises a compoundrepresented by Chemical Formula 1:

wherein, in Chemical Formula 1, R¹ to R⁸ are each independentlyhydrogen, a C1 to C3 alkyl group, a substituted or unsubstituted C4 toC20 alkyl group, a substituted or unsubstituted C3 to C20 cycloalkylgroup, a substituted or unsubstituted C6 to C20 aryl group, asubstituted or unsubstituted C3 to C20 heteroaryl group, a substitutedor unsubstituted C2 to C40 alkylamine group, or a substituted orunsubstituted C12 to C40 arylamine group, provided that at least one ofR¹ to R⁸ is a substituted or unsubstituted C4 to C20 alkyl group, X¹ andX² are independently selected from N and C(—R^(a)), and X³ and X⁴ areindependently selected from S, N—R^(b), and C(—R^(c))(—R^(d)), whereinR^(a), R^(b), R^(c), and R^(d) are independently selected from hydrogen,a substituted or unsubstituted C1 to C20 alkyl group, a substituted orunsubstituted C3 to C20 cycloalkyl group, a substituted or unsubstitutedC6 to C20 aryl group, and a substituted or unsubstituted C3 to C20heteroaryl group, L¹ and L² are independently selected from a singlebond and a substituted or unsubstituted methylene group or C2 to C4alkenylene group, and i, j, k, l, and m are each independently 0 or 1.13. The electroluminescent device of claim 1, wherein the first holetransporting material comprises at least one of compounds represented byChemical Formula 2-1 to Chemical Formula 2-2:

wherein, in Chemical Formula 2-1 to Chemical Formula 2-2, X³ and X⁴ arethe same as defined in Chemical Formula 1 of claim 2, R¹¹, R¹², R¹³,R¹⁴, R¹⁵, R¹⁶, R²¹, R²², R²³, R²⁴, R²⁵, and R²⁶ are each independentlyhydrogen, a C1 to C3 alkyl group, a substituted or unsubstituted C4 toC20 alkyl group, a substituted or unsubstituted C3 to C20 cycloalkylgroup, a substituted or unsubstituted C6 to C20 aryl group, or asubstituted or unsubstituted C3 to C20 heteroaryl group, provided thatat least one of R¹¹ to R¹⁴ and at least one of R²¹ to R²⁶ are asubstituted or unsubstituted C4 to C20 alkyl group, and R²⁷ and R²⁸ areeach independently hydrogen, a C1 to C3 alkyl group, a substituted orunsubstituted C2 to C40 alkylamine group, a substituted or unsubstitutedC12 to C40 arylamine group, or a substituted or unsubstituted carbazolylgroup.
 14. The electroluminescent device of claim 1, wherein the firsthole transporting material comprises at least one of compoundsrepresented by Chemical Formula A to Chemical Formula F:


15. The electroluminescent device of claim 1, wherein the first holetransporting material is included in an amount of greater than or equalto 2 weight percent and less than 30 weight percent based on a totalweight of the emission layer.
 16. The electroluminescent device of claim1, wherein the emission layer does not comprise cadmium, lead, or acombination thereof.
 17. The electroluminescent device of claim 1,wherein the plurality of quantum dots comprises a Group II-VI compound,a Group III-V compound, a Group IV-VI compound, a Group IV element orcompound, a Group I-III-VI compound, a Group I-II-IV-VI compound, or acombination thereof.
 18. The electroluminescent device of claim 1,wherein the electroluminescent device further comprises a hole transportlayer disposed between the emission layer and the first electrode andcomprising a second hole transporting material; an electron transportlayer disposed between the emission layer and the second electrode; or acombination thereof.
 19. The electroluminescent device of claim 15,wherein the electron transport layer comprises a plurality of inorganicnanoparticles and the inorganic nanoparticle comprises a compoundrepresented by Chemical Formula A:Zn_(1-x)M_(x)O  Chemical Formula A wherein, in Chemical Formula 1, M isMg, Ca, Zr, W, Li, Ti, Y, Al, or a combination thereof, and 0≤x≤0.5. 20.The electroluminescent device of claim 1, wherein the electroluminescentdevice has a maximum external quantum efficiency (EQE) of greater thanor equal to about 10% and the electroluminescent device has a T95 ofgreater than or equal to about 400 hours when it is operated at 1,500candela per square meter.
 21. A display device comprising theelectroluminescent device of claim 1.